Small northern white whale (6)
I believe the answer is:
beluga
'small northern white whale' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beluga that I've seen before include "Small white whale; type of caviar" , "Large caviare-yielding sturgeon" , "Large sturgeon or it's caviare" , "Could a bugle be a source of caviar?" , "'Russian sturgeon, source of caviar (6)'" .)