Cult's on-air broadcast not heard by us (10)
I believe the answer is:
ultrasonic
'not heard by us' is the definition.
'cult's on-air broadcast' is the wordplay.
'broadcast' indicates anagramming the letters.
'cults'+'on'+'air'='cultsonair'
'cultsonair' with letters rearranged gives 'ULTRASONIC'.
(Other definitions for ultrasonic that I've seen before include "Higher than can be heard" , "Involving sound waves beyond human hearing" , "Of high frequency" , "Above the limit of human hearing" .)