Mass murder participant died (5)
I believe the answer is:
crowd
'mass' is the definition.
(I know that mass is a more specific form of the action crowd)
'murder participant died' is the wordplay.
'murder participant' becomes 'crow' (I can't justify this - if you can you should believe this answer much more).
'died' becomes 'd' (abbreviation next to the year of someone's death).
'crow'+'d'='CROWD'
Can you help me to learn more?
(Other definitions for crowd that I've seen before include "Masses" , "Social set" , "Flock" , "Large number of people gathered together" , "Multitude" .)