Exposed to dry outdoors (4,3)
I believe the answer is:
open air
'outdoors' is the definition.
('open air' can be a synonym of 'outdoors')
'exposed to dry' is the wordplay.
'exposed' becomes 'open' ('open' can be a synonym of 'exposed').
'to dry' becomes 'air' (airing is a kind of drying).
'open'+'air'='OPEN-AIR'
(Other definitions for open air that I've seen before include "definitely not in the concert hall" , "Uncovered" , "Unenclosed space outdoors" , "like 12 [CONCERT] in Hyde Park" , "Out-of-doors" .)