Traffic in East Devon river heading west (5)
I believe the answer is:
trade
'traffic' is the definition.
(traffic can mean illegal trade)
'east devon river heading west' is the wordplay.
'east' becomes 'e' (synonyms).
'devon river' becomes 'dart' (River Dart in Devon).
'heading west' says the letters should be written backwards.
'e'+'dart'='edart'
'edart' back-to-front is 'TRADE'.
'in' is the link.
(Other definitions for trade that I've seen before include "Calling" , "Barter, do business" , "Business - (kind of) wind" , "Commerce - traffic" , "Dealings, business" .)