Aquatic creature (5)
I believe the answer is:
otter
'aquatic creature' is the definition.
(animal that lives by water)
This is all the clue.
(Other definitions for otter that I've seen before include "Freshwater brown carnivore" , "Freshwater mammal with dark brown fur" , "Holt dweller" , "Aquatic river mammal" , "eg Tarka" .)