Going back on decision to have transplant operation? (6,2,5)

I believe the answer is:

change of heart

'going back on decision' is the definition.

I don't understand the rest of the clue.

(Other definitions for change of heart that I've seen before include "Attitude shift" , "Having a different view" , "Profound shift in outlook" , "rethink" , "Profound alteration in outlook or opinion" .)

I've seen this clue in The Times.

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