Patterned carpet unknown in a big church (9)
I believe the answer is:
axminster
'patterned carpet' is the definition.
(type of patterned carpet)
'unknown in a big church' is the wordplay.
'unknown' becomes 'x' (algebra).
'in' is an insertion indicator.
'big church' becomes 'minster'.
'a'+'minster'='aminster'
'x' placed into 'aminster' is 'AXMINSTER'.
(Other definitions for axminster that I've seen before include "Type of carpet" , "West Country town" , "Carpeting" , "south-west town" , "Devon town; carpet" .)