Tunnel used by vehicle going west on this route (6)
I believe the answer is:
subway
'tunnel' is the definition.
(I know that subway is a type of tunnel)
'vehicle going west on this route' is the wordplay.
'vehicle' becomes 'bus' (public transport vehicle).
'going west' is a reversal indicator (letters heading to the west or left).
'on' is a charade indicator (letters next to each other).
'this route' becomes 'way' (way is a kind of route).
'bus' backwards is 'sub'.
'sub'+'way'='SUBWAY'
'used by' acts as a link.
(Other definitions for subway that I've seen before include "Passage under a road" , "New York city trains" , "crossing point" , "New York's "tube"" , "Pedestrian tunnel under a road" .)