Freshwater mammal (5)
I believe the answer is:
otter
'freshwater mammal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for otter that I've seen before include "Aquatic creature" , "fish-eater displayed" , "Freshwater brown carnivore" , "Holt dweller" , "carnivore is concerned" .)