Largest mammal (4,5)
I believe the answer is:
blue whale
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">largest 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 blue whale that I've seen before include "World's largest animal" , "there's nothing bigger" , "World's largest mammal" , "Largest living animal" .)