There's not much in potato mashing (4)

I believe the answer is:

atom

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'there's not' is the definition.
I can't tell whether this definition defines the answer.

'much in potato mashing' is the wordplay.
'much' indicates a hidden word (many of the letters in the clue).
'ATOM' can be found hidden inside 'in potato mashing'.

Can you help me to learn more?

(Other definitions for atom that I've seen before include "tiny thing" , "type of bomb" , "Tiny bit" , "Particle; molecule" , "Piece of matter in moat" .)

I've seen this clue in the Evening Standard.

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