It is not clear as to how this accident happened.
Is this kind of contruction possible?
I think this "it" is like "It is easy to do this".
Can this "as to" be inserted like that? And does it change the meaning?
PS I wrote this example.
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It is not clear as to how this accident happened.
Your sentence sounds fine to my ear. :)
As to your usage of "it", it seems as if "it" is probably being used as a dummy pronoun. A dummy pronoun has no semantic meaning, and is used purely for syntactic function. In your example, the dummy pronoun "it" is the grammatical subject.
As to your usage of the expression "as to", it seems to be standard usage. For information related to this issue, a usage dictionary can often be helpful. For instance, in my copy of Merriam-Webster's Concise Dictionary of English Usage, there is the entry "as to" on pages 99-101. They even talk specifically about the use of "as to" when used in front of the word "how":
2. One of the chief complaints made about as to is its superfluity when used in front of such conjunctions as how, why, and whether:
. . . the question as to how they should be referred to --H. L. Mencken, in Essays on Language and usage, ed. Leonard F. Dean & Kenneth G. Wilson, 2nd ed.,1963
They gave the royal stamp of approval to a work, and in doing so they provided clues as to how it might be read --Robert Darnton, The Kiss of Lamourette, 1990
. . . it should be clear as to why Joyce could find no inspiration in a cultural renaissance that found so much of theme and subject in a legendary Irish past --James T. Farrell, The League of Frightened Philistines, 1945
Also, that section ends with:
The emendation shows that as to can be omitted but does not prove that it must be omitted. It is clear that many writers find that their sentences sound better with as to retained.
Earlier in the "as to" entry (in the "1." section), there is this:
The truth is that none of the objections will stand up to comparison with evidence of actual use. As to is used in literary and general expository prose, in formal and informal settings, and in print and in speech.
MWCDEU's whole entry on "as to" might interest you, and make you feel more confident about using that expression.
That above info might be enough for you. But if you want more solid grammatical information, such as from a reference grammar, then you might be interesting in the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language (CGEL). Some info is on the following pages: page 624 ("as to" in  discussion), page 976 ("as to" in ), page 979 ("as to" in , as to being optional), page 979 ("as to" in [32.i], as to being complement to the adjective "clear"). Note that the referred pages from 976-79 are within section "5.3 Survey of constructions containing subordinate interrogatives".
I don't think OP's example is an appropriate use of as to (which in such contexts can normally be directly replaced by regarding, concerning the matter of, in respect of, etc.).
1: It is not clear how this accident happened.
2: ?It is not clear as to how this accident happened.
3: ?There is uncertainty how this accident happened.
4: There is uncertainty as to how this accident happened.
There's probably no clear-cut "grammatical rule" here, but I don't like #2 or #3 above, and I'd like them even less if as to were replaced by any of my suggested alternatives.1
It's hardly worth trying to identify an actual "thing" referenced by it - things might seem clearer if we recast #1 as "How this accident happened is unclear", but most native speakers probably just see the usage as akin to "It's raining", "There's a storm coming". As John Lawler points out in this ELU answer...
English Rule No.1 is
Every sentence must have a subject NP.
Which to my mind means it (and there) in such constructions don't really have any "meaning" at all - they're simply forced on us by the rules of grammar/syntax. It has various names, including weather "it", existential "it", existential "it", anticipatory "it", but I'd say the first of those is misleading because it applies in more contexts than just the weather; the last because often the "real" subject is never specified.
1 But see my comment below. I don't object to as to how in general, but that seems to change in constructions with only a "dummy" subject. Recasting #2 as above...
2a: ??As to how this accident happened is unclear.
2b: As to how this accident happened, it is unclear.
...I can "fix" the (to me) unacceptable #2a by re-introducing a dummy pronoun. But note that whereas the original #1 can appear as an initial utterance, #2b is only valid in an already ongoing "conversation". That's because as to means turning [now] to, and you can't do that unless the dialogue has already started (so it has a "focus" from which you can "turn away" to discuss how the accident happened).
TL;DR: Syntactically, a wh-word (what, when, where, who, which, why and how) can serve as a "subject", but as to [how, why, etc.] can't.
That use of "it" is correct. English sentences all require a subject. When there is no subject per se, we use the dummy subject "it." This is because not every situation has a "doer" - sometimes things just happen. For instance, when rain falls from the sky, we have the verb "rain" - rain happens, so we say "IT is raining."
If you were to invert this sentence, the situation itself would become the subject and you could dispense with "it":
How this accident happened is not clear.