Well ventilated (4)
I believe the answer is:
'well ventilated' is the definition.
(letting a lot of air through)
This is all the clue.
(Other definitions for airy that I've seen before include "Well-ventilated; delicate" , "Delicate; well ventilated" , "Well ventilated; insubstantial" , "Light and spacious" , "Old astronomer" .)