Farewell to the French about to expire (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'to the french about to expire' is the wordplay.
'to the french' becomes 'au' ('to the' in French).
'about' means one lot of letters goes inside another.
'to expire' becomes 'die' (synonyms).
'au' placed around 'die' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Goodbye, monsieur" , "Au revoir (to a kind friend?)" , "Farewell to France" , "Comment on leaving" , "Farewell, adios" .)