Great deal of service (4)
I believe the answer is:
mass
This clue is a double definition.
'great deal' is the first definition.
(I know that great deal can be written as mass)
'service' is the second definition.
(kind of church service)
'of' is the link.
(Other definitions for mass that I've seen before include "Matter; large amount" , "Service; horde" , "Body of matter" , "Get together" , "Quantity; religious service" .)