A lot of force and not a stone turned (7)

I believe the answer is:

megaton

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'a lot of force' is the definition.
I can't tell whether this defines the answer.

'not a stone turned' is the wordplay.
'stone' becomes 'gem' (gemstone).
'turned' is a reversal indicator.
'not'+'a'+'gem'='notagem'
'notagem' in reverse letter order is 'MEGATON'.

'and' is the link.

Can you help me to learn more?

(Other definitions for megaton that I've seen before include "Powerful explosive unit" , "it weighs a lot" , "weight" , "Measure of explosive power - montage (anag)" , "Unit of explosive power" .)

I've seen this clue in the Sydney Morning Herald.

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