Fears removing outer layer will reveal slip (5)

I believe the answer is:

error

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

I cannot understand how the rest of the clue works.

(Other definitions for error that I've seen before include "Slip-up" , "A mistake or inaccuracy" , "1 down" , "Solecism" , "Way of operating" .)

I've seen this clue in The Times.

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