Quartet heading west on a Roman road (3)
I believe the answer is:
via
'roman road' is the definition.
(I've seen this before)
'quartet heading west on a' is the wordplay.
'quartet' becomes 'iv' (roman numerals for four).
'heading west' shows that the letters should be reversed in order.
'on' means one lot of letters go next to another.
'iv' reversed gives 'vi'.
'vi'+'a'='VIA'
(Other definitions for via that I've seen before include "By way of - in Latin" , "A Roman way" , "Roman road" , "Passing through" , "By means of" .)