Referee, adjudicator (7)
I believe the answer is:
arbiter
This clue is a double definition.
'referee' is the first definition.
(I've seen this before)
'adjudicator' is the second definition.
(I've seen this before)
(Other definitions for arbiter that I've seen before include "One has the authority to settle disputes" , "Judge, final authority" , "Ref" , "Source of authority" , "Rarebit? (anag)" .)