Bess's mate quietly quits unbridled love-in (4)
I believe the answer is:
orgy
'unbridled love-in' is the definition.
(lovemaking party)
'bess's mate quietly quits' is the wordplay.
'bess's mate' becomes 'Porgy' (as in the opera Porgy and Bess).
'quietly' becomes 'p' (p for piano and quiet).
'quits' indicates named letters should be taken away.
'porgy' with 'p' removed is 'ORGY'.
(Other definitions for orgy that I've seen before include "Excess" , "Overindulgent revel" , "A licentious immoral party" , "Saturnalia" , "Bacchanalian party" .)