"Free Willy" whale (4)
I believe the answer is:
orca
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">free willy whale</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 orca that I've seen before include "Cor! a killer whale" , "Large mammal" , "Marine creature" , "Killer at sea" , "Massive killer" .)