He meets rising anger in firm for so long (7)
I believe the answer is:
cheerio
'so long' is the definition.
('cheerio' can be a synonym of 'so long')
'he meets rising anger in firm' is the wordplay.
'meets' is a charade indicator (letters next to each other).
'rising' shows that the letters should be reversed in order (going up in a down clue).
'anger' becomes 'ire' ('ire' can be a synonym of 'anger'**).
'in' is an insertion indicator.
'firm' becomes 'co' (a firm is a company).
'ire' written backwards gives 'eri'.
'he'+'eri'='heeri'
'heeri' inserted within 'co' is 'CHEERIO'.
'for' acts as a link.
(Other definitions for cheerio that I've seen before include "Say this before drinking" , "Farewell - drinking toast" , "An informal hearty goodbye" , "Informal goodbye, sounds blithe" , "Final message" .)