A mending job, dammit! (4)

I believe the answer is:

darn

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'a mending' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

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

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(Other definitions for darn that I've seen before include "Sew" , "Repair holes by sewing" , "Mend with stitches" , "Mend (woollens)" , "Repair (with needle)" .)

I've seen this clue in the Evening Standard.

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