Party involving upper-class singers? (3)
I believe the answer is:
duo
'singers?' is the definition.
(a pair of singers)
'party involving upper-class' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'involving' is an insertion indicator.
'upper-class' becomes 'u' (similar in meaning).
'do' enclosing 'u' is 'DUO'.
(Other definitions for duo that I've seen before include "Two of a kind" , "Two-person act" , "They might play" , "Performing couple" , "rejected Arab's lute" .)