Magna Carta king (4)
I believe the answer is:
john
'magna carta king' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for john that I've seen before include "With nineteen accross twentieth century US actor known for westerns, etc" , "Trick" , "Old King" , "Wayne, Denver or Elton?" , "Magna Carta king" .)