Murderer I put in prison (4)
I believe the answer is:
cain
'murderer' is the definition.
(Cain who killed Abel)
'i put in prison' is the wordplay.
'put in' indicates putting letters inside.
'prison' becomes 'can' (the can is American slang for prison**).
'i' inserted inside 'can' is 'CAIN'.
(Other definitions for cain that I've seen before include "The first known murderer" , "Biblical killer" , "Eldest son of Adam, Eve" , "Abel's brother" , "First son" .)