Come down after party in mobsters' haunt (8)
I believe the answer is:
gangland
'mobsters' haunt' is the definition.
(area where mobsters or gangs are active)
'come down after party' is the wordplay.
'come down' becomes 'land' (to land is to come down after flying).
'after' is a charade indicator (letters next to each other).
'party' becomes 'gang' (both can mean a group of people).
'land' after 'gang' is 'GANGLAND'.
'in' is the link.
(Other definitions for gangland that I've seen before include "World of organised crime" , "haunt of criminals?" , "Where to find mobsters" , "Criminal underworld" .)