Not much of a cat (4)
I believe the answer is:
atom
'not much' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'a cat' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'a'+'tom'='ATOM'
'of' is the link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Minute fragment" , "Tiny bit" , "Piece of matter in moat" , "Tiny particle, causes mighty explosion" , "Small amount of matter" .)