Murderer kidnaps prone officer (7)
I believe the answer is:
captain
'officer' is the definition.
(captain is a kind of officer)
'murderer kidnaps prone' is the wordplay.
'murderer' becomes 'cain' (Cain who killed Abel).
'kidnaps' indicates putting letters inside (I've seen this in other clues).
'prone' becomes 'apt' (I've seen this before).
'cain' placed around 'apt' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Commander of a ship" , "American police officer" , "US policeman" , "One traditionally goes down with a sinking ship" , "Ship's commander" .)