Trade in motor vehicles? (7)
I believe the answer is:
traffic
I believe this clue is a double definition.
'trade in' is the first definition.
(thesaurus)
'motor vehicles?' is the second definition.
(motor vehicles may cause road traffic)
(Other definitions for traffic that I've seen before include "Deal in, trade" , "Truck" , "Trade (in cars?)" , "Lots of vehicles" , "Business" .)