Flop as lover after heart transplant (5)
I believe the answer is:
loser
'flop' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
(Other definitions for loser that I've seen before include "Generally inept" , "One who frequently fails (colloq.)" , "One who is always a failure" , "Unsuccessful contestant" , "Roles for a non-winner" .)