Leaders of outlawed mob eternally respect this agreement (6)
I believe the answer is:
omerta
'leaders of outlawed mob eternally respect this agreement' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'leaders of outlawed mob eternally respect this agreement' is the wordplay.
'leaders of' suggests taking the first letters.
The initial letters of 'outlawed mob eternally respect this agreement' is 'OMERTA'.
(Other definitions for omerta that I've seen before include "Mafia code of silence; at Rome (anag.)" , "conspiracy of silence" , "Mafia code of silence, never inform" , "Mafia pledge of silence" , "vow of silence" .)