Judge a bishop eating snack (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I've seen this before)
'a bishop eating snack' is the wordplay.
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'eating' means one lot of letters goes inside another.
'snack' becomes 'bite' (synonyms).
'rr' enclosing 'bite' is 'rbiter'.
'a'+'rbiter'='ARBITER'
(Other definitions for arbiter that I've seen before include "One who settles disputes" , "Judge - mediator" , "expert" , "Source of authority" , "Judge, adjudicator" .)