Expert needing to make firm solvent (7)
I believe the answer is:
acetone
'solvent' is the definition.
(acetone is a kind of solvent)
'expert needing to make firm' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'needing' is a charade indicator (letters next to each other).
'to make firm' becomes 'tone' (as in toning up muscles).
'ace'+'tone'='ACETONE'
(Other definitions for acetone that I've seen before include "Common solvent" , "Industrial solvent" , "Paint thinner" , "Inflammable liquid" , "chemical is the solution" .)