Farewell bargain announced (7)
I believe the answer is:
goodbye
'farewell' is the definition.
(goodbye is a kind of farewell)
'bargain announced' is the wordplay.
'bargain' becomes 'good buy'.
'announced' indicates a 'sounds like' (homophone) clue.
'goodbuy' is a homophone of 'GOODBYE'.
(Other definitions for goodbye that I've seen before include "6 Down" , "Adieu" , "Cheerio" , "so long" , "Farewell remark" .)