Sneaky agent having change of heart (3)

I believe the answer is:

sly

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

I cannot understand how the remainder of the clue works.

(Other definitions for sly that I've seen before include "Cunning, deceitful" , "Devious, cunning" , "Wily, shifty" , "'Cunning, wily (3)'" , "Unpleasantly cunning and secretive" .)

I've seen this clue in The Times.

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