Broadcast studio sign (2,3)
I believe the answer is:
on air
'broadcast studio sign' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for on air that I've seen before include "Broadcasting live" , "If you're really happy, you'll float here" , "Currently broadcasting" , "Broadcasting at this moment" , "In the process of broadcasting on radio or TV" .)