Popular party, otherwise not held outside (6)
I believe the answer is:
indoor
'not held outside' is the definition.
I can't judge whether this definition defines the answer.
'popular party otherwise' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'party' becomes 'do' (do can informally mean a party).
'otherwise' becomes 'or'.
'in'+'do'+'or'='INDOOR'
Can you help me to learn more?
(Other definitions for indoor that I've seen before include "Situated inside" , "Located within a building" , "under one roof?" , "away from the other elements" , "Situated under cover" .)