River animal (5)
I believe the answer is:
otter
'river animal' is the definition.
(animal that may live by a river)
This is all the clue.
(Other definitions for otter that I've seen before include "Fish-eating water animal with brown fur and flat tail" , "Tarka, for one" , "bad-tempered little beast" , "Fish-eating member of weasel family" , "Aquatic river mammal" .)