A service revolution in part of India (5)
I believe the answer is:
assam
'part of india' is the definition.
('Assam' is part of 'india')
'a service revolution' is the wordplay.
'service' becomes 'mass' (church service).
'revolution' says the letters should be written backwards.
'mass' reversed gives 'ssam'.
'a'+'ssam'='ASSAM'
'in' is the link.
(Other definitions for assam that I've seen before include "part of India" , "Top quality tea" , "Asian area?" , "Indian province" , "Indian tea-producing region" .)