Number three, perhaps? (5)

I believe the answer is:

ether

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But I can't explain it at all! Could you help me understand?

(Other definitions for ether that I've seen before include "Clear upper air" , "Formerly an anaesthetic" , "Imaginary interstellar medium" , "Anaesthetic; clear sky (poetic)" , "Old anaesthetic -- three (anag)" .)

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

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