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