Son leaves showing calm acceptance, having prevailed (7)
I believe the answer is:
reigned
'prevailed' is the definition.
('reign' can be a synonym of 'prevail')
'son leaves showing calm acceptance' is the wordplay.
'son' becomes 's'.
'leaves' indicates named letters should be taken away.
'showing calm acceptance' becomes 'resigned'.
'resigned' with 's' taken away is 'REIGNED'.
'having' acts as a link.
(Other definitions for reigned that I've seen before include "Held sway" , "Was king" , "Maintained high position" , "Was monarch" , "Had sovereign power" .)