Sinner's rude tale about king (9)
I believe the answer is:
adulterer
'sinner's' is the definition.
(I've seen this before)
'rude tale about king' is the wordplay.
'about' indicates anagramming the letters.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'rude'+'tale'='rudetale'
'rudetale' anagrammed gives 'adultere'.
'adultere'+'r'='ADULTERER'
(Other definitions for adulterer that I've seen before include "One playing away" , "Away player" , "Sinner" , "one known to cheat" , "Perhaps playboy" .)