Former King stabbing Old English monster (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'former king stabbing old english' is the wordplay.
'former king' becomes 'gr' (short for Georgius Rex, King George in Latin).
'stabbing' means one lot of letters goes inside another (some letters stab themselves into others).
'old english' becomes 'OE'.
'gr' going inside 'oe' is 'OGRE'.
(Other definitions for ogre that I've seen before include "Giant of folklore, eats humans" , "Giant who eats people" , "Terrifying monster" , "Human-eating giant" , "Man-eating giant of folklore" .)