River mammal (5)
I believe the answer is:
otter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river mammal</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 otter that I've seen before include "Semi-aquatic mammal" , "Good swimmer" , "Holt dweller" , "Tarka, for one" , "Freshwater carnivorous mammal with webbed and clawed feet" .)