Final passage in European Union the French rejected (8)
I believe the answer is:
eventual
'final' is the definition.
(thesaurus)
'passage in european union the french rejected' is the wordplay.
'passage' becomes 'vent' (vent is a kind of passage**).
'in' means one lot of letters goes inside another.
'european union' becomes 'eu'.
'the french' becomes 'la'.
'rejected' says the letters should be written in reverse.
'la' reversed gives 'al'.
'vent' inserted inside 'eu' is 'eventu'.
'eventu'+'al'='EVENTUAL'
(Other definitions for eventual that I've seen before include "'Inevitable, with time (8)'" , "Happening in due course" , "Coming at last, ultimate" , "Occurring at the end, ultimate" , "Occurring after some time" .)