Vehicle not back in box (6)
I believe the answer is:
carton
'box' is the definition.
(I know that carton is a type of box)
'vehicle not back' is the wordplay.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'not back' becomes 'on' ('on' is the opposite to 'back').
'cart'+'on'='CARTON'
'in' is the link.
(Other definitions for carton that I've seen before include "Container for food or liquid" , "Box made of cardboard" , "Small, light box or container" , "Cardboard or plastic container" , "Dickensian hero" .)