Freshwater mammals (6)
I believe the answer is:
otters
'freshwater mammals' is the definition.
(I've seen this before)
This is all the clue.
(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" .)