So long, amigo! (5)
I believe the answer is:
adios
'so long amigo' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for adios that I've seen before include "Bye!" , "Goodbye (20 across)" , "Adieu, arrivederci" , "So long, amigo" , "Au revoir, amigo!" .)