French "Farewell!" (5)
'french farewell' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adieu that I've seen before include "12 Down" , "Some say it when leaving" , "Parting word" , "Farewell, Monsieur!" , "Leave" .)
(Another definition for a bientot that I've seen is " See you shortly at Cannes".)