Film outstanding gallery (7)
I believe the answer is:
acetate
'film' is the definition.
(type of material)
'outstanding gallery' is the wordplay.
'outstanding' becomes 'ace' (I've seen this before).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'ace'+'tate'='ACETATE'
(Other definitions for acetate that I've seen before include "Synthetic fabric" , "Demo disc" , "film on overhead projector" , "Sort of fabric" , "It once covered records" .)