Outmoded anesthetic (5)
I believe the answer is:
'outmoded anesthetic' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ether that I've seen before include "Anaesthetic; clear sky (poetic)" , "Imaginary interstellar medium" , "Old anaesthetic -- three (anag)" , "Type of chemical" , "number" .)