Roman road goes round southern pass (4)
I believe the answer is:
visa
'pass' is the definition.
(pass allowing international travel)
'roman road goes round southern' is the wordplay.
'roman road' becomes 'via' (I've seen this before).
'goes round' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'via' going around 's' is 'VISA'.
(Other definitions for visa that I've seen before include "Addition to passport" , "Travel document is required to enter Virginia" , "It allows the bearer to enter the country issuing it" , "Endorsement allowing bearer to enter country" , "Travel warrant" .)