Masses retiring, arriving at bar (4)
I believe the answer is:
stop
'bar' is the definition.
(thesaurus)
'masses retiring' is the wordplay.
'masses' becomes 'pots' ('pot' can be a synonym of 'mass').
'retiring' is a reversal indicator.
'pots' in reverse letter order is 'STOP'.
'arriving at' is the link.
(Other definitions for stop that I've seen before include "Go no further" , "Quit" , "It could be the organist's principal" , "port of call" , "Withhold payment" .)