Radio studio sign (2,3)

I think it could be either:

Which length description are you looking for?:

2,3

on air

'radio studio sign' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for on air that I've seen before include "very happily?" , "Broadcasting at this moment" , "How one walks when elated" , "In the act of broadcasting" , "Broadcasting live" .)

I've seen this clue in The Wall Street Journal and the USA Today.

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