Number three? Wrong (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'three? wrong' is the wordplay.
'wrong' is an anagram indicator.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "Solvent; clear sky (poet.)" , "Medium supposedly" , "Liquid used as anaesthetic/ solvent" , "Colourless, highly flammable liquid" , "Old anaesthetic -- three (anag)" .)