Couple united in party (3)
I believe the answer is:
duo
'couple' is the definition.
(synonyms)
'united in party' is the wordplay.
'united' becomes 'u'.
'in' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'u' going within 'do' is 'DUO'.
(Other definitions for duo that I've seen before include "Performing couple" , "A team of two" , "Brace" , "entertainers such as Eric and Ernie" , "Pair of musicians" .)