King Arthur's magician (6)
I believe the answer is:
merlin
'king arthur's magician' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for merlin that I've seen before include "Arthur's sorcerer" , "Arthur's adviser" , "small aggressive bird of prey" , "Hawk-like magician of Camelot" , "Arthurian wizard" .)