By way of a Roman road (3)
I believe the answer is:
via
This clue is a triple definition!
'by' is the first definition.
(travelling by or via a place)
'way' is the second definition.
(I've seen this in other clues)
'of' acts as a link.
'a roman road' is the third definition.
(I've seen this in another clue)
(Other definitions for via that I've seen before include "By way of - in Latin" , "Roman road" , "A Roman way" , "By means of" , "Going through" .)