Murderer one put in prison (4)
I believe the answer is:
cain
'murderer' is the definition.
(Cain who killed Abel)
'one put in prison' is the wordplay.
'one' becomes 'i' (Roman numeral).
'put in' is an insertion indicator.
'prison' becomes 'can' (the can is American slang for prison**).
'i' going inside 'can' is 'CAIN'.
(Other definitions for cain that I've seen before include "First murderer reported in the Bible" , "Abel's brother" , "Eldest son of Adam, Eve" , "Biblical killer" , "Adam's son" .)