Form of comedy duo’s broadcast challenge (5,2,2)
I believe the answer is:
stand up to
'challenge' is the definition.
(thesaurus)
'form of comedy duo's broadcast' is the wordplay.
'form of comedy' becomes 'stand-up' (stand-up comedy).
'duo' becomes 'two' (a duo is two people).
'broadcast' shows a homophone (sound like) (how the word might sound on the radio or TV).
'two' is a homophone of 'to'.
'standup'+'to'='STAND UP TO'
(Other definitions for stand up to that I've seen before include "Brave" , "Resist (someone)" , "Resist person courageously - get off your knees" .)