Top quality gallery material used in plastics (7)
I believe the answer is:
acetate
'material used in plastics' is the definition.
'top quality gallery' is the wordplay.
'top quality' becomes 'ace' (ace can mean excellent).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'ace'+'tate'='ACETATE'
(Other definitions for acetate that I've seen before include "Artificial textile fibre" , "Textile fibre, plastic" , "Sort of fabric" , "Transparency; textile fibre" , "Recording" .)