Top quality paint thinner (7)
I believe the answer is:
acetone
'paint thinner' is the definition.
(I've seen this before)
'top quality' is the wordplay.
'top' becomes 'ace' (ace can informally mean very good).
'quality' becomes 'tone' (synonyms).
'ace'+'tone'='ACETONE'
(Other definitions for acetone that I've seen before include "Colourless solvent" , "chemical is the solution" , "Eat once (anag) - solvent" , "Inflammable liquid" , "Industrial solvent" .)