Roman farewell (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
vale
(Other definitions for vale that I've seen before include "Glen; goodbye!" , "'''Sweet ... of Avoca, how calm could I rest'' (Moore) (4)'" , "Maida -, N. London area; farewell (archaic)" , "Valley" , "Trough between ridges" .)