There are compound words in English like well-known, ill-mannered and so on. The rules tell us that they are spelled with a hyphen (well-known) if they are attributes (attributive adjectives), but without hyphen (well known) if they are predicates (predicative adjectives). It's all right. But what if they are dictionary entries? I think these words may be present in dictionaries. Will they have a hyphen in this case?
There're no hard and fast rules whether or not we should hyphenate a compound adjective like well-known/well known. But it's far more common to hyphenate such adjectives if they are used as attributive adjectives and not to hyphenate them when used as predicative adjectives.
The Free Dictionary states:
"Well-known can be spelled with or without a hyphen. You usually spell it with a hyphen in front of a noun and without a hyphen after a verb. For examples:
I took him to a well-known doctor in Harley Street.
The building became very well known."
The general rule is that multiword descriptions using adverbs should not be hyphenated. As 'well' is an adverb, it should never be hyphenated in multiword descriptions.
However, if you consider 'well-known' to be one word, then hyphenating it would be correct in either predicate or attributive position.
Considering dictionaries vary on what they consider to be hyphenated words containing 'well' and that hyphens in general are messy and a bit unnatural for some, I personally would recommend never using a hyphen. The grammar still follows general rules for adverbs, you don't have to worry about what a 'well' word is, and it's easier to read.