Courtyards circulated oxygen at ceiling (5)
I believe the answer is:
atria
'courtyards' is the definition.
(atrium is a kind of courtyard)
'circulated oxygen at ceiling' is the wordplay.
'circulated' says the letters should be written in reverse (I've seen 'circulating' mean this).
'oxygen' becomes 'air' (air contains oxygen).
'at ceiling' becomes 'ta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'air'+'ta'='airta'
'airta' backwards is 'ATRIA'.
Can you help me to learn more?
(Other definitions for atria that I've seen before include "Upper chambers of the heart" , "Central courtyards" , "Central courts, halls" , "Central courts (in ancient Rome or shopping arcades)" , "Apartments" .)