Freshwater mammal (5)
I believe the answer is:
otter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">freshwater mammal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for otter that I've seen before include "Aquatic creature" , "fish-eater displayed" , "Freshwater brown carnivore" , "Holt dweller" , "carnivore is concerned" .)