Bit of a cat (4)
I believe the answer is:
atom
'bit' 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 "A tiny particle of matter with a big bang" , "Very small object or amount" , "Smallest particle in an element able to take part in a chemical reaction" , "Smallest amount" , "Piece of matter in moat" .)