Bess's partner lacking power in party (4)
I believe the answer is:
orgy
'party' is the definition.
(thesaurus)
'bess's partner lacking power' is the wordplay.
'bess's partner' becomes 'porgy' (ref to opera Porgy and Bess).
'lacking' indicates named letters should be taken away.
'power' becomes 'p'.
'porgy' with 'p' taken away is 'ORGY'.
'in' is the link.
(Other definitions for orgy that I've seen before include "Bacchanalian party" , "Riotous revel" , "Unrestrained revel" , "Wild, debauched gathering" , "debauched scene" .)