Number three, possibly? (5)

I believe the answer is:

ether

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'number' is the definition.
(anaesthetic, something which numbs)

I don't understand how the remainder 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" .)

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