Church mostly dubious about material (7)
I believe the answer is:
chiffon
'material' is the definition.
(chiffon is a kind of material)
'church mostly dubious about' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'mostly' means to remove the last letter.
'dubious' becomes 'iffy' (I've seen this before).
'about' becomes 'on' (**).
'iffy' with its final letter removed is 'iff'.
'ch'+'iff'+'on'='CHIFFON'
(Other definitions for chiffon that I've seen before include "Fine transparent fabric" , "Sheer silk or rayon fabric" , "Light silk/nylon fabric" , "Fine (semi)transparent fabric" , "Fine material" .)