Disabled beggar's topless sex party (4)
I believe the answer is:
orgy
'sex party' is the definition.
(group sex event)
'disabled beggar's topless' is the wordplay.
'disabled beggar' becomes 'Porgy' (character in the Gershwin opera Porgy and Bess).
'topless' means to remove the first letter.
'porgy' with its initial letter removed is 'ORGY'.
(Other definitions for orgy that I've seen before include "Wild licentious party" , "Wild activity" , "Rumpy-pumpy party" , "Debauched party" , "Frenzy; bacchanalia" .)