Being broadcast (2,3)
I believe the answer is:
on air
'being broadcast' is the definition.
(on air on radio or TV)
This is the entire clue.
(Other definitions for on air that I've seen before include "In the act of broadcasting" , "Broadcasting studio sign" , "In the process of broadcasting - floating aloft?" , "In the process of broadcasting on radio or TV" , "now showing" .)