A lot of people in service (4)
I believe the answer is:
mass
I believe this clue is a double definition.
'a lot of people' is the first definition.
(mass of people)
'service' is the second definition.
(church service)
'in' acts as a link.
(Other definitions for mass that I've seen before include "Get together" , "Bulk - RC service" , "Total" , "13 [BULK]" , "seasonal midnight service?" .)