Number three is wrong (5)

I believe the answer is:

ether

'number' is the definition.
(I've seen this before)

'three is wrong' is the wordplay.
'is wrong' indicates an anagram.
'three' is an anagram of 'ETHER'.

(Other definitions for ether that I've seen before include "Type of chemical" , "Medium supposedly" , "Solvent; clear sky (poet.)" , "Former anaesthetic" , "Clear upper air" .)

I've seen this clue in The Telegraph and The Globe and Mail.

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