Declaration nun once meant to be broadcast (12)
I believe the answer is:
announcement
'declaration' is the definition.
(I've seen this before)
'nun once meant to be broadcast' is the wordplay.
'to be broadcast' indicates anagramming the letters.
'nun'+'once'+'meant'='nunoncemeant'
'nunoncemeant' with letters rearranged gives 'ANNOUNCEMENT'.
(Other definitions for announcement that I've seen before include "Public declaration" , "Public notice" , "Proclamation" , "Statement, declaration" , "A public statement" .)