Judge, umpire (7)
I believe the answer is:
arbiter
I believe this is a double definition.
'judge' is the first definition.
(I know that arbiter is a type of judge)
'umpire' is the second definition.
(I know that umpire can be written as arbiter)
(Other definitions for arbiter that I've seen before include "One judges a disputed issue" , "One decides in disputes" , "Source of authority" , "One must exercise judgment" , "Ref" .)