Third man sounds competent enough (4)
I believe the answer is:
abel
'third man' is the definition.
(biblical third man after Adam and Cain)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for abel that I've seen before include "The first murderee?" , "Cain's younger brother" , "Early murder victim" , "Victim of 11" , "One man was murdered" .)