Blemishing the exam papers? (7)

I believe the answer is:

marking

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'blemishing' is the definition.
(marking is a kind of blemishing)

I can't explain the rest of the clue.

(Other definitions for marking that I've seen before include "Assessing script" , "Staying close to an opposing player" , "Labelling" , "assessment" , "Rating (school work)" .)

I've seen this clue in the Evening Standard.

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