The clear, upper air; an anaesthetic (5)
I believe the answer is:
ether
This clue is a double definition.
'the clear upper air' is the first definition.
(upper air in Greek philosophy)
'an anaesthetic' is the second definition.
(ether is a kind of anaesthetic)
(Other definitions for ether that I've seen before include "Anaesthetic liquid; clear sky" , "Medium supposedly" , "Volatile organic liquid once used as a general anaesthetic" , "Gas - air - space" , "Anaesthetic is, strangely, there" .)