Three is odd number (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'three is odd' is the wordplay.
'is odd' is an anagram indicator.
'three' anagrammed gives 'ETHER'.
(Other definitions for ether that I've seen before include "Anaesthetic is, strangely, there" , "An early anaesthetic" , "Colourless volatile highly inflammable liquid" , "It could knock you out" , "Liquid used as anaesthetic/ solvent" .)