Quite a while back I remember there being a shortcut key for the action "Clear all items in current list". Cant find it now. Has it been removed? Can shortcut keys be created by users?
Though if you have specified a particular directory, it will relist the files in that directory rather then clearing all items from the list. Clearing the directory line & hitting 'Return' (at that point), sets the directory list to 'Computer' (depending on OS), which should then leave you with a blank list.