People enter for this sort of sport (6)
I believe the answer is:
indoor
'sport' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for indoor that I've seen before include "Taking place within a building" , "Situated within a building - maybe a sport" , "Within walls" , "Practised inside" , "covered" .)