One in prison. He killed his brother (4)
I believe the answer is:
cain
'brother' is the definition.
(I have seen 'Brother of Abel ' mean 'cain' so perhaps 'brother' could also mean 'cain')
'one in prison' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'prison' becomes 'can' (the can is American slang for prison).
'i' put within 'can' is 'CAIN'.
'he killed his' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for cain that I've seen before include "Brother of Abel" , "First person born in Bible" , "Abel's brother" , "Biblical killer" , "Eldest son of Adam, Eve" .)