First killer takes in clever leader (7)
I believe the answer is:
captain
'leader' is the definition.
(I know that captain is a type of leader)
'first killer takes in clever' is the wordplay.
'first killer' becomes 'cain' (Cain who killed Abel. I am not sure about the 'first' bit.).
'takes in' means one lot of letters goes inside another.
'clever' becomes 'apt' (synonyms).
'cain' going around 'apt' is 'CAPTAIN'.
Can you help me to learn more?
(Other definitions for captain that I've seen before include "eg Hornblower" , "soldier" , "Skipper" , "Naval commander" , "One traditionally goes down with a sinking ship" .)