Judge a champ right to corner leader of rebels (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I've seen this before)
'a champ right to corner leader of rebels' is the wordplay.
'champ' becomes 'bite' (I've seen this before).
'right' becomes 'r' (common abbreviation).
'to corner' means one lot of letters goes inside another (inserted letters get trapped or cornered).
'leader of' suggests taking the first letters.
The first letter of 'rebels' is 'r'.
'a'+'bite'+'r'='abiter'
'abiter' going around 'r' is 'ARBITER'.
(Other definitions for arbiter that I've seen before include "expert" , "Mediator or referee" , "One has the authority to resolve disputes" , "Judge - mediator" , "Source of authority" .)