Old number?
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">old number?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(chemical historically used to numb patients)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ether that I've seen before include "Colourless liquid used as a solvent" , "Imaginary interstellar medium" , "It could knock you out" , "Chemical solvent" , "Volatile organic liquid once used as a general anaesthetic" .)