Lowland farewell (4)
I believe the answer is:
vale
I believe this is a double definition.
'lowland' is the first definition.
(I've seen this in another clue)
'farewell' is the second definition.
(I've seen this before)
(Other definitions for vale that I've seen before include "Long depression containing a river" , "Glen; goodbye!" , "Maida -, N. London area; farewell (archaic)" , "Cheers" , "Valley" .)