Vehicle not packed in a box! (6)
I believe the answer is:
carton
'a box' is the definition.
(I know that carton is a type of box)
'vehicle not packed' is the wordplay.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'not packed' becomes 'on' (I can't justify this - if you can you should believe this answer much more).
'cart'+'on'='CARTON'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for carton that I've seen before include "Box without great strength" , "Cardboard or plastic container" , "Flapped cardboard box" , "Container for food or liquid" , "A cardboard container as for milk" .)