A mistake to catch you out? (4)

I believe the answer is:

slip

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'a mistake' is the definition.
(a slip is a blunder or mistake)

I can't explain the rest of the clue.

(Other definitions for slip that I've seen before include "Cutting" , "Underwear" , "Lose footing or make mistake" , "Slight mistake" , "Loose one's footing" .)

I've seen this clue in the Evening Standard.

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