Spanish "So long" (5)
I believe the answer is:
adios
'spanish so long' 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 "Au revoir, amigo!" , "Spanish farewell" , "Goodbye to Spain" , "Farewell amigo!" , "Good-bye (Sp.)" .)