Very naughty boy born our bastard High King (5,4)
I believe the answer is:
brian boru
'high king' is the definition.
(11th-century High King of Ireland)
'very naughty boy born our bastard' is the wordplay.
'very naughty boy' becomes 'Brian' (reference to Monty Python's Life of Brian).
'born' becomes 'b' (abbreviation next to the year of someone's birth).
'bastard' indicates an anagram (bastard can mean impure or mixed).
'our' with letters rearranged gives 'oru'.
'brian'+'b'+'oru'='BRIAN BORU'
(Other definitions for brian boru that I've seen before include "Irish king killed at the battle of Contarf" , "King killed by Danes at the Battle of Clontarf" .)