First murderer takes in clever leader (7)
I believe the answer is:
captain
'leader' is the definition.
(captain is a kind of leader)
'first murderer takes in clever' is the wordplay.
'first murderer' becomes 'cain' (I've seen this before).
'takes in' means one lot of letters goes inside another.
'clever' becomes 'apt' (synonyms).
'cain' placed around 'apt' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Is Pat Cain in charge of the ship?" , "Lead (a team)" , "Skipper" , "Commander of a ship" , "Naval commander" .)