Duly ends broadcast without warning (8)
I believe the answer is:
suddenly
'without warning' is the definition.
(thesaurus)
'duly ends broadcast' is the wordplay.
'broadcast' is an anagram indicator.
'duly'+'ends'='dulyends'
'dulyends' anagrammed gives 'SUDDENLY'.
(Other definitions for suddenly that I've seen before include "Out of the blue" , "Quickly and without warning" , "Quickly, unexpectedly" , "In an unexpected manner" , "Happening without warning" .)