Farewell from a girl attached to Brussels? (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'a girl attached to brussels?' is the wordplay.
'a girl' becomes 'adi' (**).
'attached to' means one lot of letters go next to another.
'brussels?' becomes 'eu' (Brussels is a metonym for the European Union).
'adi'+'eu'='ADIEU'
'from' acts as a link.
(Other definitions for adieu that I've seen before include "Cheerio" , "Goodbye, monsieur" , "Parting word" , "Comment on leaving" , "Adios, Arrivederci" .)