Box is not overturned under vehicle (6)
I believe the answer is:
carton
'box' is the definition.
(I know that carton is a type of cardboard box)
'not overturned under vehicle' is the wordplay.
'overturned' says the letters should be written in reverse.
'under' means one lot of letters go next to another (below, in a down clue).
'vehicle' becomes 'car' (car is a kind of vehicle).
'not' reversed gives 'ton'.
'ton' put after 'car' is 'CARTON'.
'is' is the link.
(Other definitions for carton that I've seen before include "Box without great strength" , "Flapped cardboard box" , "Sydney -, Tale of Two Cities hero" , "Cardboard or plastic container" , "A cardboard container as for milk" .)