Farm vehicle attached to box (6)
I believe the answer is:
carton
'box' is the definition.
(carton is a kind of box)
'farm vehicle attached' is the wordplay.
'farm vehicle' becomes 'cart' (I've seen this before).
'attached' becomes 'on' (I've seen this in other clues).
'cart'+'on'='CARTON'
'to' acts as a link.
(Other definitions for carton that I've seen before include "fictional lawyer" , "Container for food or liquid" , "Cantor (anag)" , "He did a far, far better thing" , "A cardboard container as for milk" .)