Murderer grabs appropriate leader (7)
I believe the answer is:
captain
'leader' is the definition.
(I know that captain is a type of leader)
'murderer grabs appropriate' is the wordplay.
'murderer' becomes 'cain' (Cain who killed Abel).
'grabs' is an insertion indicator.
'appropriate' becomes 'apt' (synonyms).
'cain' enclosing 'apt' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "One traditionally goes down with a sinking ship" , "Chief" , "eg Hornblower" , "Leader" , "US policeman" .)