XmListDeleteAllItems man page
XmListDeleteAllItems — A List function that deletes all items from the list
#include <Xm/List.h> void XmListDeleteAllItems( Widget widget);
XmListDeleteAllItems deletes all items from the list.
Specifies the ID of the List from whose list the items are deleted
For a complete definition of List and its associated resources, see XmList(3).