Judge rarebit to be ruined (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I know that arbiter is a type of judge)
'rarebit to be ruined' is the wordplay.
'to be ruined' indicates an anagram.
'rarebit' anagrammed gives 'ARBITER'.
(Other definitions for arbiter that I've seen before include "Judge, adjudicator" , "Rear bit (anag.)" , "One has the authority to resolve disputes" , "One appointed to settle a dispute" , "Judge, umpire" .)