Adieu, so long (8)
I believe the answer is:
farewell
This clue is a double definition.
'adieu' is the first definition.
(I've seen this before)
'so long' is the second definition.
(both mean goodbye)
(Other definitions for farewell that I've seen before include "symphony" , "Goodbye, adieu, so long" , "Valedictory word" , "Goodbye, hope you get on" , "Goodbye, cheerio" .)