Trendy parties -- outside or not outside (7)
I believe the answer is:
indoors
'not outside' is the definition.
('indoors' is the opposite to 'outside')
'trendy parties outside or' is the wordplay.
'trendy' becomes 'in'.
'parties' becomes 'dos' (do is a kind of party).
'outside' means one lot of letters goes inside another.
'dos' enclosing 'or' is 'doors'.
'in'+'doors'='INDOORS'
(Other definitions for indoors that I've seen before include "Within the four walls" , "under cover" , "Inside a building" , "Covered" , "safe from the weather" .)