Church walls in Barcelona perhaps quartz (8)
I believe the answer is:
basilica
'church' is the definition.
(I know that basilica is a type of church)
'walls in barcelona perhaps quartz' is the wordplay.
'walls in' suggests removing the centre.
'perhaps quartz' becomes 'silica' (quartz is a type of silica).
'barcelona' with its centre taken out is 'ba'.
'ba'+'silica'='BASILICA'
(Other definitions for basilica that I've seen before include "Important Roman Catholic church" , "Roman building used for public administration" , "Magnificent church" , "Church edifice" , "A large Christian church with two rows of columns (8)" .)