Old number?
I believe the answer is:
'old number?' is the definition.
(chemical historically used to numb patients)
This is the entire clue.
(Other definitions for ether that I've seen before include "Colourless liquid used as a solvent" , "Imaginary interstellar medium" , "It could knock you out" , "Chemical solvent" , "Volatile organic liquid once used as a general anaesthetic" .)