Rich Tea provided in westbound service area (5)
I believe the answer is:
assam
'rich tea' is the definition.
(assam is a type of tea)
'westbound service area' is the wordplay.
'westbound' is a reversal indicator (letters written from right to left).
'service' becomes 'mass' (kind of church service).
'area' becomes 'a' (maths abbreviation).
'mass'+'a'='massa'
'massa' back-to-front is 'ASSAM'.
'provided in' is the link.
(Other definitions for assam that I've seen before include "Tea-growing state in north-east India" , "State in northeastern India" , "An Indian part" , "Variety of tea" , "Leaves to get in hot water" .)