Freshwater mammals (6)
I believe the answer is:
otters
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">freshwater mammals</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 otters that I've seen before include "Water animals with webbed feet and brown fur" , "Mammals with webbed feet" , "Sleek short-furred swimmers" , "River mammals" , "Sleek water-animals" .)