Former anaesthetic (5)
I believe the answer is:
ether
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">former anaesthetic</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ether that I've seen before include "Type of chemical" , "Clear upper air" , "Liquid used as anaesthetic/ solvent" , "Highly flammable liquid once used as an anaesthetic" , "What's clear" .)