Clear upper air; solvent (5)
I believe the answer is:
ether
I believe this clue is a double definition.
'clear upper air' is the first definition.
(upper air in Greek philosophy)
'solvent' is the second definition.
(I've seen this before)
(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" .)