Farm vehicle atop box (6)
I believe the answer is:
carton
'box' is the definition.
(I know that carton is a type of box)
'farm vehicle atop' is the wordplay.
'farm vehicle' becomes 'cart' (I've seen this in another clue).
'atop' becomes 'on'.
'cart'+'on'='CARTON'
(Other definitions for carton that I've seen before include "Cantor (anag)" , "fictional lawyer" , "Container for food or liquid" , "Dickensian hero" , "Sydney -, Tale of Two Cities hero" .)