Ordinary enclosure, dry out of doors
I believe the answer is:
open air
'out of doors' is the definition.
(I've seen this before)
'ordinary enclosure dry' is the wordplay.
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'dry' becomes 'air' (airing is a kind of drying).
'o'+'pen'+'air'='OPEN-AIR'
(Other definitions for open air that I've seen before include "Alfresco" , "like Glastonbury gigs" , "definitely not in the concert hall" , "Unenclosed space outdoors" , "like 12 [CONCERT] in Hyde Park" .)