Revenge, possibly result of change of heart (3,3,3)

I believe the answer is:

tit for tat

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'revenge' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for tit for tat that I've seen before include "Retaliation for an injury - almost equal" , "An eye for an eye" , "... pork-pie maybe" , "Equivalent retaliation" , "You hit me, I hit you" .)

I've seen this clue in The Times and The Sunday Times.

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