Gallery supporting excellent film (7)
I believe the answer is:
acetate
'film' is the definition.
(type of material)
'gallery supporting excellent' is the wordplay.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'excellent' becomes 'ace' (ace can informally mean very good).
'tate' put after 'ace' is 'ACETATE'.
(Other definitions for acetate that I've seen before include "Sort of fabric" , "Material in old record" , "Synthetic fabric" , "Artificial textile fibre" , "Transparent film" .)