Greeting or farewell (4)
I believe the answer is:
ciao
I believe this clue is a double definition.
'greeting' is the first definition.
(I've seen this before)
'farewell' is the second definition.
(I've seen this before)
(Other definitions for ciao that I've seen before include "Informal greeting (Italian)" , "Hi - bye" , "See you in Italy" , "Italian's so long" , "European greeting" .)