Eric is beheaded by cruel king (7)
I believe the answer is:
richard
'king' is the definition.
(Richard I or Richard II)
'eric is beheaded by cruel' is the wordplay.
'is beheaded' means to remove the first letter (the head of the word is removed).
'by' is a charade indicator (letters next to each other).
'cruel' becomes 'hard' (similar in meaning).
'eric' with its first letter taken off is 'ric'.
'ric'+'hard'='RICHARD'
(Other definitions for richard that I've seen before include "Cliff --; -- Wagner" , "Lionheart king" , "-- Wagner, composer" , "Branson or Cliff" , "Mr Hannay" .)