Caviar source (6)
I believe the answer is:
beluga
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">caviar source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)