Nitrogen associated with the clear upper air, or lower? (6)
I believe the answer is:
nether
'lower?' is the definition.
(as in nether regions)
'nitrogen associated with the clear upper air' is the wordplay.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'associated with' says to put letters next to each other.
'the clear upper air' becomes 'ether' (upper air in Greek philosophy).
'n'+'ether'='NETHER'
'or' acts as a link.
(Other definitions for nether that I've seen before include "North European country" , "inferior" , "Underground" , "These regions are beneath" , "Located below" .)