Weasel's relative (5)
I believe the answer is:
otter
'weasel's relative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for otter that I've seen before include "River animal" , "Good swimmer" , "Playful aquatic animal of 'Tarka' fame" , "Sleek river mammal" , "Freshwater brown carnivore" .)