Tiny part of a cat (4)
I believe the answer is:
atom
'tiny part' is the definition.
(I've seen this in another clue)
'a cat' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'a'+'tom'='ATOM'
'of' acts as a link.
(Other definitions for atom that I've seen before include "It was once thought to be smallest particle of matter" , "Minute particle" , "A really tiny piece of something" , "A mighty physical particle or bomb" , "Small particle of matter" .)