Latin farewell (4)
I believe the answer is:
vale
'latin farewell' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vale that I've seen before include "Glen; goodbye!" , "Trough between ridges" , "Cheers" , "'''Sweet ... of Avoca, how calm could I rest'' (Moore) (4)'" , "Maida -, N. London area; farewell (archaic)" .)