Idiot singer knocking bishop out (3)

I believe the answer is:

ass

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'idiot' is the definition.
(both can mean a stupid person)

I don't understand the remainder of the clue.

(Other definitions for ass that I've seen before include "Wally" , "Horse hybrid" , "One easily had" , "twit!" , "mug" .)

I've seen this clue in The Times.

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