Campaigner’s appeal accepted by simple king (8)
I believe the answer is:
crusader
'campaigner's' is the definition.
(I've seen this before)
'appeal accepted by simple king' is the wordplay.
'appeal' becomes 'SA' (old-fashioned abbreviation for sex appeal).
'accepted by' indicates putting letters inside.
'simple' becomes 'crude' (crude can mean simple or unsophisticated).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'sa' going within 'crude' is 'crusade'.
'crusade'+'r'='CRUSADER'
(Other definitions for crusader that I've seen before include "Richard I, for instance" , "Campaigner for a cause, like Richard the Lionheart" , "Medieval expeditionary" , "One campaigning vigorously" , "Champion" .)