Horror doesn't start by mistake (5)

I believe the answer is:

error

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'mistake' is the definition.
(I know that mistake can be written as error)

I cannot understand how the remainder of the clue works.

(Other definitions for error that I've seen before include "Mistake, lapse" , "Miscalculation" , "Mistake or fault" , "wrongdoing" , "wrong" .)

I've seen this clue in The Mirror.

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