Paint solvent (7)
'paint solvent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acetone that I've seen before include "Eat once (anag) - solvent" , "Common solvent" , "Inflammable liquid" , "Volatile stuff" , "chemical is the solution" .)
(Other definitions for thinner that I've seen before include "Solvent; less thick" , "what one adds to paint" , "Slimmer" , "Diluting solvent" , "Volatile liquid" .)