Officer liable to be kidnapped by a murderer (7)
I believe the answer is:
captain
'officer' is the definition.
(captain is a kind of officer)
'liable to be kidnapped by a murderer' is the wordplay.
'liable' becomes 'apt' ('apt' can be a synonym of 'liable').
'to be kidnapped by' means one lot of letters goes inside another.
'a murderer' becomes 'cain' (Cain who killed Abel).
'apt' inserted within 'cain' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "eg Hook, Cook" , "American police officer" , "Naval commander" , "Leader" , "US policeman" .)