Some clergy attack centres enforcing vow of silence … (6)
I believe the answer is:
omerta
'vow of silence' is the definition.
(mafia code of silence)
'some clergy attack centres' is the wordplay.
'some clergy' becomes 'omer' (I can't justify this - if you can you should believe this answer much more).
'centres' indicates the central letters.
The centre of 'attack' is 'ta'.
'omer'+'ta'='OMERTA'
'enforcing' acts as a link.
(Other definitions for omerta that I've seen before include "conspiracy of silence" , "Mafia pledge of silence" , "Mafia code of silence; at Rome (anag.)" , "Mafia code of silence, never inform" , "vow of silence" .)