Crossword Genius

Vehicle not packed in a box! (6)

Ross

I believe the answer is:

carton

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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