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Isn't saying you can't do the job because you're busy just an excuse?
Literal
Busy [because-から] that work [subject-が] can't-do [quotative-という] [nominalizer-の] [topic-は] mere excuse [isn't-it-じゃないか].
A complex, layered sentence. という nominalizes the preceding quote ('the claim that you can't do it because you're busy'). のは...じゃないか frames a rhetorical question challenging that claim. 言い逃れ (excuse/evasion) is a compound of 言い (saying) + 逃れ (escape). 単なる (mere) is an attributive form only used before nouns — the rentaikei of a na-adjective-like word.