Box vehicle put on weight (6)
I believe the answer is:
carton
'box' is the definition.
(I know that carton is a type of box)
'vehicle put on weight' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'put on' means one lot of letters go next to another.
'weight' becomes 'ton' (unit of weight).
'car'+'ton'='CARTON'
(Other definitions for carton that I've seen before include "Dickensian hero" , "Flapped cardboard box" , "Box made of cardboard" , "fictional lawyer" , "Container for food or liquid" .)