Chief doesn't complete mistake (5)

I believe the answer is:

gaffe

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'mistake' is the definition.
(a gaffe is a mistake or error)

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

(Other definitions for gaffe that I've seen before include "indiscreet remark" , "Clanger" , "Indiscretion" , "bad mistake" , "embarrassing mistake" .)

I've seen this clue in The Mirror.

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