The best player is a defender (8)
I believe the answer is:
champion
This clue is a double definition.
'the best player' is the first definition.
(eg a world champion)
'defender' is the second definition.
(I know that champion is a type of defender)
'is a' is the link.
(Other definitions for champion that I've seen before include "Winner - advocate" , "Support (cause); winner" , "Overall winner" , "Competitor who has defeated all others" , "Defender of a cause" .)