Regressive way of paying someone, one crossing a line (4)

I believe the answer is:

scab

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'one crossing a line' is the definition.
(I've seen this in another clue)

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

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(Other definitions for scab that I've seen before include "evidence of recent injury" , "Crust on wound, or a blackleg" , "cover" , "Scoundrel" , "work when others are out" .)

I've seen this clue in The Independent.

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