Murderer has appropriate officer (7)
I believe the answer is:
captain
'officer' is the definition.
(I know that captain is a type of officer)
'murderer has appropriate' is the wordplay.
'murderer' becomes 'cain' (Cain who killed Abel).
'has' means one lot of letters goes inside another.
'appropriate' becomes 'apt' (synonyms).
'cain' going around 'apt' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Army rank" , "Lead (a team)" , "eg Hornblower" , "US policeman" , "Leader" .)