Brother's killer — one In US prison (4)
I believe the answer is:
cain
'brother's killer' is the definition.
'cain' can be an answer for 'killer' (Cain who killed Abel). I am not certain of the 'brother's' bit.
'one in us prison' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'us prison' becomes 'can' (informal term for a prison in US English).
'i' placed inside 'can' is 'CAIN'.
Can you help me to learn more?
(Other definitions for cain that I've seen before include "Brother of Abel" , "The first known murderer" , "Abel's brother" , "Slayer of Abel" , "Eldest son of Adam, Eve" .)