Assemble for service (4)
I believe the answer is:
mass
This clue is a double definition.
'assemble' is the first definition.
(thesaurus)
'service' is the second definition.
(I've seen this in other clues)
'for' is the link.
(Other definitions for mass that I've seen before include "Large number of people" , "Substance; a service" , "Accumulation" , "Religious ceremony" , "Celebration of the Eucharist, especially in the RC Church" .)