Criminal cur undergoing change of heart (6)

I believe the answer is:

robber

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'criminal' is the definition.
(I know that criminal can be written as robber)

I don't understand the rest of the clue.

(Other definitions for robber that I've seen before include "Violent thief" , "One who thieves" , "Property stealer" , "Mugger" , "Burglar" .)

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

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