Solitary guy has change of heart: he’s not successful (5)
I believe the answer is:
loser
'solitary guy has change of heart he's not successful' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'heart he's not successful' is the wordplay.
'heart' means to look at the middle letters.
'he's' becomes 'bills' (short for William).
'not' becomes 'o' (I've seen this in other clues).
'successful' becomes 'ser' (I am not sure about this - if you are sure you should believe this answer much more).
The middle of 'bills' is 'l'.
'l'+'o'+'ser'='LOSER'
(Other definitions for loser that I've seen before include "sad type?" , "One who frequently fails (colloq.)" , "Generally inept" , "one that's been left behind" , "Non-achiever" .)