A service revolution in part of India (5)
I believe the answer is:
'part of india' is the definition.
('Assam' is part of 'india')
'a service revolution' is the wordplay.
'service' becomes 'mass' (I've seen this in other clues).
'revolution' shows that the letters should be reversed in order.
'mass' written backwards gives 'ssam'.
'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" .)