Farewell announcement that is socially acceptable (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'announcement that is socially acceptable' is the wordplay.
'announcement' becomes 'ad' (short for advert).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'socially acceptable' becomes 'u'.
'ad'+'ie'+'u'='ADIEU'
(Other definitions for adieu that I've seen before include "Comment on leaving" , "Bye" , "Goodbye, madame" , "See you later" , "I'm away (in France?)" .)