Expert goes to the gallery for a chemical (7)
I believe the answer is:
acetate
'for a chemical' is the definition.
(thesaurus)
'expert goes to the gallery' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'goes to' means one lot of letters go next to another.
'the gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'ace'+'tate'='ACETATE'
(Other definitions for acetate that I've seen before include "Visual aid" , "It once covered records" , "(Chemical used in production of) clear film" , "Transparency; textile fibre" , "Sort of fabric" .)