A month's broadcasts are astounding! (5)
I believe the answer is:
amaze
'are astounding' is the definition.
(to amaze is to astound)
'a month's broadcasts' is the wordplay.
'month's' becomes 'may's' (May is an example).
'broadcasts' shows a homophone (sound like) (I've seen 'broadcast' mean this (how the word might sound on the radio or TV)).
'mays' is a homophone of 'maze'.
'a'+'maze'='AMAZE'
(Other definitions for amaze that I've seen before include "Bewilder" , "Bowl over" , "Floor" , "Greatly surprise, astound" , "baffle" .)