A little bit of a cat (4)
I believe the answer is:
atom
'a little bit' is the definition.
(tiny particle)
'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 "Very small bit" , "Minute fragment" , "A tiny powerful particle" , "Particle; molecule" , "Minute particle" .)