Air at high altitudes (8)
I believe the answer is:
rarefied
'air at high altitudes' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rarefied that I've seen before include "Less dense, like the upper air" , "Air at high altitudes" , "(Of gas) at very low pressure" , "Select" , "Exalted; exclusive; thin" .)