Large marine mammal (5)
I believe the answer is:
whale
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large marine mammal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for whale that I've seen before include "Largest animal" , "Huge mammal" , "Maybe humpback" , "Mammoth marine mammal" , "It breathes through a blowhole" .)