More than a ton weight! (5)

I believe the answer is:

stone

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'ton weight' is the definition.
'stone' can be an answer for 'weight' (stone is a kind of weight). I am unsure of the 'ton' bit.

'more than a' is the wordplay.
'more than' becomes 'st' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' becomes 'one'.
'st'+'one'='STONE'

Can you help me to learn more?

(Other definitions for stone that I've seen before include "Gem; Staffs. town" , "Kernel" , "Seed of e.g. cherry" , "Rock; fruit seed" , "Rock; cherry seed" .)

I've seen this clue in the Evening Standard.

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