Old anaesthetic (5)
I believe the answer is:
ether
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">old anaesthetic</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ether that I've seen before include "Anaesthetic liquid; clear sky" , "Colourless liquid used as a solvent" , "I'm volatile" , "Gas - air - space" , "Solvent; clear sky (poet.)" .)