Herb deployed CIA in church (8)
I believe the answer is:
basilica
'church' is the definition.
(type of large church)
'herb deployed cia in' is the wordplay.
'herb' becomes 'basil' (basil is a herb).
'deployed' indicates anagramming the letters.
'in' is an insertion indicator.
'cia' anagrammed gives 'ica'.
'basil' put within 'ica' is 'BASILICA'.
(Other definitions for basilica that I've seen before include "Roman style of building" , "A large Christian church with two rows of columns (8)" , "Important Roman Catholic church" , "Romanesque church" , "Roman public administration building Christian church" .)