Winter stoat (6)
I believe the answer is:
ermine
'winter stoat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ermine that I've seen before include "Animal with brown fur that turns white in winter" , "garb for nobility" , "Fur-bearing mammal" , "Winter fur of the stoat" , "White fur of stoat" .)