It's farewell to the French as one departs a rejected European Union (5)
I believe the answer is:
adieu
'it's farewell to the french' is the definition.
(I've seen this before)
'one departs a rejected european union' is the wordplay.
'one' becomes 'i' (Roman numeral).
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'rejected' is a reversal indicator.
'european union' becomes 'eu'.
'i'+'d'+'a'='ida'
'ida' in reverse letter order is 'adi'.
'adi'+'eu'='ADIEU'
'as' acts as a link.
(Other definitions for adieu that I've seen before include "See you later" , "Fare thee well!" , "Farewell, ami!" , "Farewell (French)" , "Goodbye, madame" .)