Some number subtracted from lower number (5)

I believe the answer is:

ether

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

I cannot 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 "Clear upper air" , "Anaesthetic is, strangely, there" , "Former anaesthetic" , "Colourless volatile highly inflammable liquid" , "Formerly an anaesthetic" .)

I've seen this clue in The Times.

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