21 down
Excuse
a
sexual
impulse,
but
not
act
(5)
Excuse a sexual impulse, but not act (5)
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The definition is at the beginning of the clue.
How the clue works
Right, so Excuse is your definition, pointing to ALIBI. For the wordplay, start with A, then bolt on LIBIDO (a sexual impulse), and now whip out DO (that's your "act") from the end of it — LIBIDO minus DO leaves you LIBI. Stick A and LIBI back together and you've got your answer.