You could just add a listener for each of scenario. Something like this:
import flash.events.MouseEvent;
inputText_txt.addEventListener(MouseEvent.CLICK, clearComboBox);
comboBox.addEventListener(Event.CHANGE, clearTextBox);
function clearComboBox(event:MouseEvent):void
{
comboBox.selectedItem = -1;
}
function clearTextBox(e:Event):void
{
inputText_txt.text = "";
}