Bargain reported for so long (7)
I believe the answer is:
goodbye
'for so long' is the definition.
(synonyms)
'bargain reported' is the wordplay.
'bargain' becomes 'good buy' (I've seen this in another clue).
'reported' indicates a 'sounds like' (homophone) clue.
'goodbuy' is a homophone of 'GOODBYE'.
(Other definitions for goodbye that I've seen before include "Cheerio" , "Farewell remark" , "parting" , "Adieu" , "so long" .)