Crossword Genius

Not much of a cat (4)

Ross

I believe the answer is:

atom

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'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" .)

I've seen this clue in the Evening Standard.
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