King beginning to attack last rebels in the north (9)
I believe the answer is:
athelstan
'king' is the definition.
(I know that Athelstan is a king)
'beginning to attack last rebels in the north' is the wordplay.
'beginning to' says to take the initial letters.
'rebels' indicates anagramming the letters.
'in' indicates putting letters inside.
'north' becomes 'N' (abbreviation).
The first letter of 'attack' is 'a'.
'last' is an anagram of 'lsta'.
'the'+'n'='then'
'lsta' placed into 'then' is 'thelstan'.
'a'+'thelstan'='ATHELSTAN'
(Other definitions for athelstan that I've seen before include "King of Mercia and West Saxons" , "King in the 10th century" , "Historic English king" , "Early English king" , "Anglo-Saxon king" .)