Acid complaint by the winner (6)
I believe the answer is:
acetic
'acid' is the definition.
(acetic acid is a kind of acid)
'complaint by the winner' is the wordplay.
'complaint' becomes 'tic' (a tic is a medical complaint).
'by' is a charade indicator (letters next to each other).
'the winner' becomes 'ace' (I can't explain this - if you can you should believe this answer much more).
'tic' put after 'ace' is 'ACETIC'.
Can you help me to learn more?
(Other definitions for acetic that I've seen before include "Saucy'" , "tart" , "Like the acid in vinegar" , "Like vinegar's acid" , "really sour" .)