Medium's car? (7)
I believe the answer is:
vehicle
This clue is a double definition.
'medium's' is the first definition.
(vehicle is a kind of medium)
'car?' is the second definition.
(car is a kind of vehicle)
(Other definitions for vehicle that I've seen before include "Channel - conveyance" , "Conveyance of people or things" , "Conveyance for transporting passengers on land" , "Lorry perhaps" , "Mode of transport" .)