Expert to make firm solvent (7)

I believe the answer is:

acetone

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'solvent' is the definition.
(acetone is a kind of solvent)

'expert to make firm' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'to make firm' becomes 'tone' (I can't justify this - if you can you should give a lot more credence to this answer).
'ace'+'tone'='ACETONE'

Can you help me to learn more?

(Other definitions for acetone that I've seen before include "Volatile stuff" , "Paint thinner" , "chemical is the solution" , "it dissolves" , "Common solvent" .)

I've seen this clue in The Sun.

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