Horror doesn't start to slip (5)

I believe the answer is:

error

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

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

(Other definitions for error that I've seen before include "heresy" , "Trial and ___" , "A mistake or inaccuracy" , "Fallacy, mistake" , "Blunder or mistake" .)

I've seen this clue in The Mirror.

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