Card at gallery refers to material used in photographer's film (7)
I believe the answer is:
acetate
'film' is the definition.
(type of material)
'card at gallery' is the wordplay.
'card' becomes 'ace' (ace is a type of playing card).
'at' is a charade indicator (letters next to each other).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'ace'+'tate'='ACETATE'
'refers to material used in photographer's' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for acetate that I've seen before include "(Chemical used in production of) clear film" , "Transparency; textile fibre" , "Transparent film" , "Visual aid" , "Demo disc" .)