May need mending, darn it! (4)

I believe the answer is:

sock

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'darn it' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

(Other definitions for sock that I've seen before include "Article to cover extremity" , "Hit; garment" , "Hit hard; woollen item" , "Foot covering" , "Item of footwear" .)

I've seen this clue in the Evening Standard.

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