Thinner expert has style (7)
I believe the answer is:
acetone
'thinner' is the definition.
(acetone is a kind of thinner)
'expert has style' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'has' means one lot of letters go next to another.
'style' becomes 'tone' (tone is a kind of style).
'ace'+'tone'='ACETONE'
(Other definitions for acetone that I've seen before include "Remover of nail polish" , "Paint thinner" , "chemical is the solution" , "Colourless solvent" , "Volatile stuff" .)