Murderer: one kept in jail (4)
I believe the answer is:
cain
'murderer' is the definition.
(Cain who killed Abel)
'one kept in jail' is the wordplay.
'one' becomes 'i' (Roman numeral).
'kept in' is an insertion indicator.
'jail' becomes 'can' (the can is American slang for prison).
'i' placed into 'can' is 'CAIN'.
(Other definitions for cain that I've seen before include "Abel's brother" , "First son" , "First person born in Bible" , "The first known murderer" , "Biblical killer" .)