Unexpectedly tedious in the open air (7)
I believe the answer is:
outside
'the open air' is the definition.
(thesaurus)
'unexpectedly tedious' is the wordplay.
'unexpectedly' is an anagram indicator.
'tedious' is an anagram of 'OUTSIDE'.
'in' is the link.
(Other definitions for outside that I've seen before include "Where smokers now often must go" , "In the fresh air?" , "Not indoors" , "Alfresco" , "having emerged from the tunnel?" .)