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