Fool who doubles in for a killer (3)

I believe the answer is:

ass

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

I don't understand how the remainder of the clue works.

(Other definitions for ass that I've seen before include "Horse-type animal" , "Bottom, temporarily" , "Stupid person or American bum" , "Long-eared animal" , "Clot" .)

I've seen this clue in The Globe and Mail.

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