Sign at a radio station (2,3)

I think it could be either:

Which length description are you looking for?:

2,3

on air

'sign at a radio station' 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 "very happily?" , "In the act of broadcasting" , "In the process of broadcasting on radio or TV" , "How one walks when elated" , "now showing" .)

I've seen this clue in the USA Today and the Universal.

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