Synthetic textile in a church gallery (7)
I believe the answer is:
acetate
'synthetic textile' is the definition.
'a church gallery' is the wordplay.
'church' becomes 'ce' (abbreviation for Church of England).
'gallery' becomes 'tate' (I've seen this before).
'a'+'ce'+'tate'='ACETATE'
'in' acts as a link.
(Other definitions for acetate that I've seen before include "Material in old record" , "Sort of fabric" , "Transparency used with an overhead projector" , "Recording" , "Synthetic fabric" .)