Brussels supports aid arrangement for so long (5)
I believe the answer is:
adieu
'for so long' is the definition.
(both can mean goodbye)
'brussels supports aid arrangement' is the wordplay.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'arrangement' indicates anagramming the letters.
'aid' with letters rearranged gives 'adi'.
'eu' put after 'adi' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Farewell, Monsieur!" , "Adios, Arrivederci" , "farewell message" , "Ciao, in French" , "Farewell (French)" .)