A great deal of service (4)
I believe the answer is:
mass
I believe this clue is a double definition.
'a great deal' is the first definition.
('mass' can be a synonym of 'great deal')
'service' is the second definition.
(kind of church service)
'of' acts as a link.
(Other definitions for mass that I've seen before include "Roman Catholic service" , "Holy service" , "seasonal midnight service?" , "state, in short" , "load" .)