Three used anaesthetic (5)

I believe the answer is:

ether

'anaesthetic' is the definition.
(ether is a kind of anaesthetic)

'three used' is the wordplay.
'used' is an anagram indicator.
'three' is an anagram of 'ETHER'.

(Other definitions for ether that I've seen before include "Imaginary interstellar medium" , "number" , "Old anaesthetic -- three (anag)" , "Chemical solvent" , "Clear upper air" .)

I've seen this clue in The Mirror.

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