Out-of-doors (6)
'out-of-doors' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for open-air that I've seen before include "not below roof" , "Alfresco" , "Outdoor" , "Taking place outdoors" , "outside" .)
(Other definitions for nature that I've seen before include "Disposition, temperament" , "Disposition; temper" , "Character - our Mother" , "mother, so-called" , "Kind - disposition" .)