Box of significant weight next to vehicle (6)
I believe the answer is:
carton
'box' is the definition.
(I know that carton is a type of box)
'significant weight next to vehicle' is the wordplay.
'significant weight' becomes 'ton' (unit of weight. I am not sure about the 'significant' bit.).
'next to' says to put letters next to each other.
'vehicle' becomes 'car' (car is a kind of vehicle).
'ton' after 'car' is 'CARTON'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for carton that I've seen before include "Flapped cardboard box" , "Box without great strength" , "Sydney -, Tale of Two Cities hero" , "A cardboard container as for milk" , "fictional lawyer" .)