'Farewell' bill island's presented by Brussels (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'bill island's presented by brussels' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'island' becomes 'i'.
'presented' is a charade indicator (letters next to each other).
'by' is a charade indicator (letters next to each other).
'brussels' becomes 'eu' (Brussels is a metonym for the European Union).
'ad'+'i'+'eu'='ADIEU'
(Other definitions for adieu that I've seen before include "I'm going" , "Au revoir (to a kind friend?)" , "Farewell (French)" , "Parting word" , "Farewell, Monsieur!" .)