The third man (4)
I believe the answer is:
abel
'the third man' is the definition.
(biblical third man after Adam and Cain)
This is the entire clue.
(Other definitions for abel that I've seen before include "Cain's murdered brother" , "The first recorded murder victim" , "Gardener's son" , "Cain's victim; - Magwitch (Great Expectations)" , "Adamson" .)