Chiefly murderers and felons in alliance (5)
I believe the answer is:
mafia
'chiefly murderers and felons in alliance' is the definition.
I can't judge whether this defines the answer.
'chiefly murderers and felons in alliance' is the wordplay.
'chiefly' indicates taking the first letters.
The initial letters of 'murderers and felons in alliance' is 'MAFIA'.
(Other definitions for mafia that I've seen before include "Group exerting a hidden sinister influence" , "Italian criminal organisation" , "Criminal organisation also known as Cosa Nostra" , "Sicilian criminals" , "connection claimed to 23 [JFK]" .)