20 down
Idiot
stops
benefit
–
blow
it!
(7)
Idiot stops benefit – blow it! (7)
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The definition is at the end of the clue.
How the clue works
The definition is blow it!, referring to the bassoon, a wind instrument you blow. The wordplay puts ASS (idiot) inside BOON (benefit) — ASS "stops" BOON — giving B-ASS-OON.