Web-footed mammals (6)
I believe the answer is:
otters
'web-footed mammals' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for otters that I've seen before include "Sleek short-furred swimmers" , "Fish-eating mammals" , "Water animals with webbed feet and flat tails" , "web-footed creatures" , "Dark brown freshwater mammals" .)