Opposite of outdoor (6)
I believe the answer is:
indoor
'opposite of outdoor' is the definition.
('indoor' is the opposite to 'outdoor')
This is the entire clue.
(Other definitions for indoor that I've seen before include "Sport like snooker, darts or badminton" , "Practised inside" , "Not outside the home" , "Within walls" , "(Of sports) performed under a roof" .)