Interstate rig (4)
I believe the answer is:
semi
'interstate rig' 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 "Prefix denoting half" , "The last but one round" , "half would do!" , "Prefix meaning half or partly" , "One of two houses with a common wall" .)