Service means a great deal (4)
I believe the answer is:
mass
I believe this clue is a double definition.
'service' is the first definition.
(religious service)
'a great deal' is the second definition.
(I know that slew can be written as mass)
'means' is the link.
(Other definitions for mass that I've seen before include "Body of matter" , "Christian ritual" , "Throng" , "Religious ceremony" , "Church service - collection" .)