Back a winner (8)
I believe the answer is:
champion
This clue is a double definition.
'back' is the first definition.
(championing is a kind of backing)
'a winner' is the second definition.
(thesaurus)
(Other definitions for champion that I've seen before include "Champ" , "Person who has defeated all others in a competition" , "The first-placed in a competition" , "Stick up for" , "Winner of first place" .)