Party includes acceptable couple (3)
I believe the answer is:
duo
'couple' is the definition.
('duo' can be a synonym of 'couple')
'party includes acceptable' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'includes' means one lot of letters goes inside another.
'acceptable' becomes 'u' ((Film classification)).
'do' going around 'u' is 'DUO'.
(Other definitions for duo that I've seen before include "Pair (of people, things)" , "Pair of singers" , "entertainers such as Eric and Ernie" , "Pair of musicians" , "Couple, pair" .)