Murderer's caught fitting lead (7)
I believe the answer is:
captain
'lead' is the definition.
(captaining is a kind of leading)
'murderer's caught fitting' is the wordplay.
'murderer' becomes 'cain' (Cain who killed Abel).
'caught' means one lot of letters goes inside another.
'fitting' becomes 'apt' (I've seen this before).
'cain' going around 'apt' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Army rank" , "Is Pat Cain in charge of the ship?" , "soldier" , "US policeman" , "One traditionally goes down with a sinking ship" .)