Judge cooking of rarebit (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I know that arbiter is a type of judge)
'cooking of rarebit' is the wordplay.
'cooking of' is an anagram indicator (I've seen 'cooking' mean this (cook can mean to falsify or distort)).
'rarebit' anagrammed gives 'ARBITER'.
(Other definitions for arbiter that I've seen before include "Someone with the power to settle the matter" , "Mediator or referee" , "Rear bit (anag.)" , "Judge, adjudicator" , "Settler of disputes" .)