Broadcasting planes fly thus (2,3,3)

I believe the answer is:

on the air

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'broadcasting' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for on the air that I've seen before include "Anti-hero engaged in live broadcast" , "radio listeners can now hear?" , "Broadcasting (hence audible)" , "In the process of broadcasting, radio or TV" .)

I've seen this clue in the Sydney Morning Herald.

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