Bank payment reversed for blackleg (4)

I believe the answer is:

scab

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'for blackleg' is the definition.
(synonyms)

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

(Other definitions for scab that I've seen before include "sign of wound healing" , "Someone who works during a strike" , "complaint with scales" , "part of healing process" , "Dried surface of wound" .)

I've seen this clue in The Sun.

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