River mammal (5)
I believe the answer is:
otter
'river mammal' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)