Fur is small fit (5)
I believe the answer is:
sable
'fur' is the definition.
(sable is a kind of fur)
'small fit' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fit' becomes 'able' ('able' can be similar in meaning to 'fit').
's'+'able'='SABLE'
'is' is the link.
(Other definitions for sable that I've seen before include "Brush" , "Black; antelope" , "Bales (anag.)" , "Adventurous courage" , "Bales of rich fur" .)