Feature of an Everest ascent, in which one may disappear (4,3)
I believe the answer is:
thin air
'in which one may disappear' is the definition.
I cannot understand how the remainder of the clue works.
(Other definitions for thin air that I've seen before include "Lack of atmosphere (from which the unexpected appears?)" , "What one vanishes into?" , "Stratosphere" , "In which things vanish, magically" , "one may disappear into?" .)