Conspiracy of silence breaks at Rome (6)
I believe the answer is:
omerta
'conspiracy of silence' is the definition.
(I've seen this in another clue)
'breaks at rome' is the wordplay.
'breaks' indicates an anagram.
'at'+'rome'='atrome'
'atrome' with letters rearranged gives 'OMERTA'.
(Other definitions for omerta that I've seen before include "conspiracy of silence" , "Mafia pledge of silence" , "Mafia code of silence, never inform" , "vow of silence" , "Mafia code of silence; at Rome (anag.)" .)