Feline with old fur (4)
I believe the answer is:
coat
'fur' is the definition.
(an animal's coat)
'feline with old' is the wordplay.
'feline' becomes 'cat' (cat is a kind of feline).
'with' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cat' placed around 'o' is 'COAT'.
(Other definitions for coat that I've seen before include "Apply layer" , "Outer covering" , "Overgarment" , "Layer of paint" , "Fleece" .)