Box adding weight to vehicle (6)
I believe the answer is:
carton
'box' is the definition.
(I know that carton is a type of box)
'weight to vehicle' is the wordplay.
'weight' becomes 'ton' (unit of weight).
'to' means one lot of letters go next to another (I've seen this in other clues).
'vehicle' becomes 'car' (car is a kind of vehicle).
'ton' put after 'car' is 'CARTON'.
'adding' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for carton that I've seen before include "Sydney -, Tale of Two Cities hero" , "Box without great strength" , "A cardboard container as for milk" , "Cardboard or plastic container" , "fictional lawyer" .)