Officer liable to be taken in by murderer (7)
I believe the answer is:
captain
'officer' is the definition.
(I've seen this before)
'liable to be taken in by murderer' is the wordplay.
'liable' becomes 'apt' ('apt' can be a synonym of 'liable').
'to be taken in by' means one lot of letters goes inside another.
'murderer' becomes 'cain' (Cain who killed Abel).
'apt' inserted inside 'cain' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "soldier" , "Skip" , "One traditionally goes down with a sinking ship" , "Is Pat Cain in charge of the ship?" , "Naval commander" .)