Open-air lobbies (5)
I believe the answer is:
atria
'open-air lobbies' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for atria that I've seen before include "Roofed central courts" , "Open courts of Roman houses" , "Open-roofed entrance halls (Roman) - glass-roofed central halls (modern)" , "Apartments" , "Central courts (in ancient Rome or shopping arcades)" .)