Ear is boxed by Rupert, mostly over source of caviar (6)
I believe the answer is:
beluga
'source of caviar' is the definition.
(I've seen this in another clue)
'ear is boxed by rupert mostly over' is the wordplay.
I cannot really understand how this works, but
'ear' could be 'lug' (informal term for an ear) and 'lug' is found in the answer.
'is' could be 'be' and 'be' is present in the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for beluga that I've seen before include "Small white whale; type of caviar" , "Could a bugle be a source of caviar?" , "Type of whale; sturgeon" , "Great Russian sturgeon - Arctic whale" , "'Russian sturgeon, source of caviar (6)'" .)