A large number in service (4)
I believe the answer is:
mass
I believe this clue is a double definition.
'a large number' is the first definition.
(I've seen this before)
'service' is the second definition.
(church service)
'in' acts as a link.
(Other definitions for mass that I've seen before include "Church rite; large group" , "Throng" , "Body of matter" , "Quantity; religious service" , "Total" .)