Vehicle in traffic (5)
I believe the answer is:
truck
I believe this is a double definition.
'vehicle' is the first definition.
(truck is a kind of vehicle)
'traffic' is the second definition.
(truck traffic is a kind of traffic)
'in' is the link.
(Other definitions for truck that I've seen before include "commercial vehicle" , "Lorry - wagon" , "Freight vehicle" , "Goods vehicle" , "Bargain" .)