Certain duet bound to be broadcast (9)
I believe the answer is:
undoubted
'certain' is the definition.
(thesaurus)
'duet bound to be broadcast' is the wordplay.
'to be broadcast' is an anagram indicator (letters scattered or cast around).
'duet'+'bound'='duetbound'
'duetbound' anagrammed gives 'UNDOUBTED'.
(Other definitions for undoubted that I've seen before include "Not questioned by anyone" , "Indisputable" , "Without question" , "Certainly genuine" .)