Rig on the road (4)
I believe the answer is:
semi
'rig on the road' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for semi that I've seen before include "Type of house (UK) - type of truck (US)" , "House linked to another" , "The next to final round" , "One of two houses with a common wall" , "House attached to one other" .)