Stoat in mere swimming (6)
I believe the answer is:
ermine
'stoat' is the definition.
(I've seen this before)
'in mere swimming' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'in'+'mere'='inmere'
'inmere' is an anagram of 'ERMINE'.
(Other definitions for ermine that I've seen before include "Winter fur of the stoat" , "Fur for an English judge" , "Fur-bearing mammal" , "Regal fur" , "White fur of weasel" .)