Large number gather in church service (4)
I believe the answer is:
mass
This clue is a quadruple definition!!
'large number' is the first definition.
(I've seen this before)
'gather in' is the second definition.
(thesaurus)
'church' is the third definition.
(thesaurus)
'service' is the fourth definition.
(kind of church service)
(Other definitions for mass that I've seen before include "Roman Catholic service" , "Great deal" , "Pound, maybe" , "Lump of material" , "Christian ritual" .)