Number three, possibly? (5)
I believe the answer is:
'number' is the definition.
(anaesthetic, something which numbs)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ether that I've seen before include "Some were put out by this" , "Once widely-used anaesthetic" , "Clear upper air" , "What's clear" , "Chemical solvent" .)