Number three is odd (5)
I believe the answer is:
ether
'number' is the definition.
(anaesthetic, something which numbs)
'three is odd' is the wordplay.
'is odd' is an anagram indicator.
'three' is an anagram of 'ETHER'.
(Other definitions for ether that I've seen before include "Formerly an anaesthetic" , "Colourless liquid used as a solvent" , "Anaesthetic; clear sky (poetic)" , "There goes the anaesthetic" , "Former anaesthetic" .)