Bess's partner didn't start the party! (4)
I believe the answer is:
orgy
'party' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for orgy that I've seen before include "Wild self-indulgent party" , "Very intimate get-together" , "Bacchanal" , "Licentious revel" , "Frenzy; bacchanalia" .)