Let's get this fixed.
In this example, "fixed" has no tense. "Get" has no tense. The only verb with tense is "let".
This "let" is in the imperative mode, present tense, active voice and indefinite aspect. It forms a predicate that takes an implicit second-person subject. It licenses a direct object and an object complement. The object is a contracted "us", and the complement is the infinitive phrase "[to] get this fixed".
The verb to let licenses bare infinitive object complements. Other verbs license full infinitives for that same role, such as:
Allow us to get this fixed.
We've got to get this fixed.
The verb to get also licenses direct objects and object complements. In these examples, the demonstrative pronoun "this" serves as the object and the participle "fixed" serves as the complement.
Most verbs have past-tense forms and so-called past-participle forms that are identical. A few, such as to do, have distinct forms:
*We've got to get this did.
We've got to get this done.
The past-tense form "did" does not work here. The participle form "done", on the other hand, is a suitable candidate for an object complement licensed by the verb to get.
You're looking for tense where tense does not exist. The "fixed" of your original and the "done" of my clarifying example are both non-finite forms. They are participles in an attributive role, modifying a direct object through a governing verb's license. The instances of "get" in these examples are also non-finite forms. They are infinitives in a substantive role, acting as the objects of their governing verbs.