A sturgeon (6)
I believe the answer is:
beluga
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a sturgeon</span>' is the definition.<br/ ><span class="explanation-format__explanation">(beluga is a kind of sturgeon)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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)'" .)