Old King George English? A monster! (4)
I believe the answer is:
ogre
'a monster' is the definition.
('ogre' can be a synonym of 'monster')
'old king george english?' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'king george' becomes 'gr' (Georgius Rex).
'english?' becomes 'e' (abbreviation).
'o'+'gr'+'e'='OGRE'
(Other definitions for ogre that I've seen before include "Monster, troll" , "Giant who eats people" , "ugly one" , "Brute" , "Monstrous creature" .)