Old murderer holding suitable officer (7)
I believe the answer is:
captain
'officer' is the definition.
(I know that captain is a type of officer)
'old murderer holding suitable' is the wordplay.
'old murderer' becomes 'cain' (Biblical figure. I am not sure about the 'old' bit.).
'holding' means one lot of letters goes inside another.
'suitable' becomes 'apt' (apt means appropriate or fitting).
'cain' going around 'apt' is 'CAPTAIN'.
Can you help me to learn more?
(Other definitions for captain that I've seen before include "eg Hook, Cook" , "eg Hornblower" , "One traditionally goes down with a sinking ship" , "Ship's commander" , "Commander of a ship" .)