A sturgeon (6)
I believe the answer is:
beluga
'a sturgeon' is the definition.
(beluga is a kind of sturgeon)
This is the entire clue.
(Other definitions for beluga that I've seen before include "Type of whale; sturgeon" , "fish" , "Great Russian sturgeon - Arctic whale" , "Species of sturgeon" , "'Russian sturgeon, source of caviar (6)'" .)