Couple hosting posh party (3)
I believe the answer is:
duo
'couple' is the definition.
('duo' can be a synonym of 'couple')
'hosting posh party' is the wordplay.
'hosting' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'party' becomes 'do' (do is a kind of party).
'u' going into 'do' is 'DUO'.
(Other definitions for duo that I've seen before include "They might play" , "Pair of singers" , "rejected Arab's lute" , "Brace" , "Music for two" .)