Slip made when urchin loses head (5)

I believe the answer is:

error

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'slip' is the definition.
(slip can mean an error or mistake)

I can't explain the rest of the clue.

(Other definitions for error that I've seen before include "Howler" , "Blunder or mistake" , "A mistake or inaccuracy" , "Faux pas" , "Bloomer, blunder" .)

I've seen this clue in The Times.

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