Pay back mean knight for running away, primarily (6)

I believe the answer is:

avenge

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'pay back' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for avenge that I've seen before include "Retaliate on behalf of" , "Pay back for" , "To seek vengeance" , "eg Nave (anag.)" , "Exact retribution on behalf of (someone)" .)

I've seen this clue in The Independent.

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