King of an unruly horde (5)
I believe the answer is:
herod
'king' is the definition.
(I know that Herod is a king)
'an unruly horde' is the wordplay.
'an unruly' indicates anagramming the letters.
'horde' with letters rearranged gives 'HEROD'.
'of' acts as a link.
(Other definitions for herod that I've seen before include "New Testament king" , "Cruel king" , "Old king" , "Infanticidal king" , "Royal baby-killer" .)