Assemble for a religious service (4)
I believe the answer is:
mass
This clue is a double definition.
'assemble' is the first definition.
(I know that mass is a more specific form of the action crowd)
'for a religious service' is the second definition.
(I've seen this before)
(Other definitions for mass that I've seen before include "Religious celebration" , "Roman Catholic service" , "Get together" , "Service; horde" , "what 13 15A [STANDARD MODEL] supposes 12 [HIGGS BOSON] gives other 5A [PARTICLES]s" .)