Boss hasn't finished -- that's a mistake (5)

I believe the answer is:

gaffe

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'that's a mistake' is the definition.
(gaffe is a kind of mistake)

I don't understand the rest of the clue.

(Other definitions for gaffe that I've seen before include "embarrassing mistake" , "A foolish error" , "Remark one wishes unmade" , "Boob" , "embarrassing error" .)

I've seen this clue in The Telegraph.

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