Number three's in a mess (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'three's in a mess' is the wordplay.
'in a mess' indicates an anagram.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "Imaginary interstellar medium" , "Old anaesthetic -- three (anag)" , "Once widely-used anaesthetic" , "Colourless liquid used as a solvent" , "Volatile solvent" .)