Atmosphere in Cairo (3)
I believe the answer is:
air
'atmosphere' is the definition.
(I know that atmosphere can be written as air)
'in cairo' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AIR' is hidden in the letters of 'cairo'.
(Other definitions for air that I've seen before include "Ventilate or give expression to" , "show" , "Of which 78% is nitrogen" , "Means of travelling" , "Publish" .)