Unpleasant feeling arising after change of heart (7)
I believe the answer is:
noisome
'unpleasant' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
(Other definitions for noisome that I've seen before include "Nasty" , "Smelling badly" , "injurious to health" , "Evil-smelling, objectionable (7)" , "Evil-smelling - sounds loud" .)