Best player's back (8)
I believe the answer is:
champion
This clue is a double definition.
'best player's' is the first definition.
(eg a world champion)
'back' is the second definition.
(champion can mean to endorse or back)
(Other definitions for champion that I've seen before include "Tournament winner" , "Advocate, defender" , "Support (cause); winner" , "Finest" , "Campaigner or winner" .)