First murderer one in prison (4)
I believe the answer is:
cain
'first murderer' is the definition.
(I've seen this in another clue)
'one in prison' is the wordplay.
'one' becomes 'i' (Roman numeral).
'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" , "Adam's son" , "Slayer of Abel" , "Eve's firstborn" , "Abel's brother" .)