Central area in a building open to the sky (6)
I believe the answer is:
atrium
'central area in a building open to the sky' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for atrium that I've seen before include "Entrance hall" , "Central hall of shopping mall etc" , "Central area in a building, open or with a glass roof" , "Central court of ancient Roman house" , "Central hall or court" .)