You succeeded with sweetheart, promoting love (6)
I believe the answer is:
solver
'you' is the definition.
(the solver of this crossword puzzle!)
'succeeded with sweetheart promoting love' is the wordplay.
I cannot really see how this works, but
'succeeded' could be 's' (genealogical abbreviation) and 's' is found in the answer.
'sweetheart' could be 'e' (the heart/centre of 'sweet') and 'e' is found in the answer.
'love' could be 'o' (In tennis, 'love' means 'zero') and 'o' is located in the answer.
This explanation may well be incorrect...
(Other definitions for solver that I've seen before include "Private Eye" , "You find" , "Crossword completer" , "One finds the answer to a mystery" , "One succeeding with crossword" .)