Clear upper air; solvent (5)
I believe the answer is:
ether
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">clear upper air</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(upper air in Greek philosophy)</span><br/ ><br/ >'<span class="explanation-format__original">solvent</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for ether that I've seen before include "Liquid used as anaesthetic/ solvent" , "The gas is there, oddly" , "number" , "It could knock you out" , "Once widely-used anaesthetic" .)