It's of signal necessity in radio (6)

I believe the answer is:

aerial

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'in radio' is the definition.
(I know that aerial is part of radio receiver)

I can't explain the rest of the clue.

(Other definitions for aerial that I've seen before include "Operating aloft" , "Device to send and receive radio signals" , "Ethereal" , "Transmitting structure" , "Signal receiver" .)

I've seen this clue in the Evening Standard.

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