King Arthur’s leader among men in combat (9)
I believe the answer is:
agamemnon
'king' is the definition.
(mythical Greek king)
'arthur's leader among men in combat' is the wordplay.
'leader' suggests taking the first letters.
'in combat' indicates anagramming the letters.
The first letter of 'arthurs' is 'a'.
'among'+'men'='amongmen'
'amongmen' is an anagram of 'gamemnon'.
'a'+'gamemnon'='AGAMEMNON'
(Other definitions for agamemnon that I've seen before include "Leader of Greeks in Trojan War" , "king of old" , "Legendary king" , "Legendary hero" , "mythological king" .)