Permit silence to be broken (7)
I believe the answer is:
license
'permit' is the definition.
('license' can be a synonym of 'permit')
'silence to be broken' is the wordplay.
'to be broken' indicates an anagram.
'silence' anagrammed gives 'LICENSE'.
(Other definitions for license that I've seen before include "Grant official permit to sell or trade" , "Authorise (something)" , "Authorise to sell in silence" , "Allow" , "Grant formal permission (to)" .)