Caviar source (6)
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I believe the answer is:
beluga
'caviar source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for beluga that I've seen before include "Small northern white whale" , "Large caviare-yielding sturgeon" , "Large Russian sturgeon" , "Species of sturgeon" , "Great Russian sturgeon - Arctic whale" .)