Brother killed by Cain (4)
I believe the answer is:
abel
'brother killed by cain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for abel that I've seen before include "Cain's 11" , "tragic figure" , "Cain's victim; - Magwitch (Great Expectations)" , "He was slain by Cain" , "Son of Adam and Eve" .)