The one in jail 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')
'the one in jail' is the wordplay.
'the one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'jail' becomes 'can' (the can is American slang for prison).
'i' inserted inside 'can' is 'CAIN'.
'killed his' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for cain that I've seen before include "The first known murderer" , "Eldest son of Adam, Eve" , "Biblical killer" , "Adam's son" , "Abel's brother" .)