Farewell to the French about to go west (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'to the french about to go west' is the wordplay.
'to the french' becomes 'au'.
'about' indicates putting letters inside.
'to go west' becomes 'die' (to go west can informally mean to die).
'au' going around 'die' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "left, making this?" , "Goodbye, madame" , "A valediction" , "Parting greeting" , "Adios, Arrivederci" .)