Number three failing (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'three failing' is the wordplay.
'failing' is an anagram indicator.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "number" , "Clear upper air" , "Colourless volatile highly inflammable liquid" , "Colourless, highly flammable liquid" , "Gas - air - space" .)