Not much of a mouser maybe? (4)
I believe the answer is:
atom
'not much' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'a mouser maybe?' is the wordplay.
'mouser maybe?' becomes 'tom' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'tom'='ATOM'
'of' is the link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "A mighty particle, makes huge explosion" , "Minute fragment" , "Smallest particle in an element able to take part in a chemical reaction" , "Something very small perhaps" , "type of bomb" .)