Back winner (8)
I believe the answer is:
champion
I believe this clue is a double definition.
'back' is the first definition.
(champion can mean to endorse or back)
'winner' is the second definition.
(I know that champion is a type of winner)
(Other definitions for champion that I've seen before include "gain position on top" , "Advocate, defender" , "Support (cause); winner" , "Fight for" , "Winner of first place" .)