Leader of school committee held by murderer (7)
I believe the answer is:
captain
'leader' is the definition.
(captain is a kind of leader)
'school committee held by murderer' is the wordplay.
'school committee' becomes 'pta' (abbreviation for parent-teacher association).
'held by' is an insertion indicator.
'murderer' becomes 'cain' (Cain who killed Abel).
'pta' placed inside 'cain' is 'CAPTAIN'.
'of' is the link.
(Other definitions for captain that I've seen before include "US policeman" , "eg Hook, Cook" , "Army rank" , "Naval rank" , "Skip" .)