Assemble for religious service (4)
I believe the answer is:
mass
I believe this clue is a double definition.
'assemble' is the first definition.
(I know that mass is a more specific form of crowd)
'religious service' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for mass that I've seen before include "Church service - collection" , "congregate for > service" , "Throng" , "13 [BULK]" , "Matter; large amount" .)