Old number three substituted (5)
I believe the answer is:
ether
'old number' is the definition.
(I've seen this in another clue)
'three substituted' is the wordplay.
'substituted' indicates an anagram (I've seen 'substitute' mean this).
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "Liquid used as anaesthetic/ solvent" , "What's clear" , "Imaginary interstellar medium" , "There goes the anaesthetic" , "It could knock you out" .)