Use the .DESTRUCT of a CLASS,THREAD
Clarion ThreadMonitor class - used to Notify a thread as each thread starts and ends (github.com)
INCLUDE('ThreadMonitor.inc'),ONCE !<-- threaded instance
ThreadMonitor ctThreadMonitor,THREAD
ThreadMonitor.ConstructEvent( 0 ) !<-- disable
ThreadMonitor.DestructEvent (Event:ThreadEnded)
ThreadMonitor.ThreadToInform( AppFrame{prop:Thread} )
! where Event:ThreadEnded is an Event:App+N