make a movieclip visible when an object is dragged onto it

Something like the following might work (not tested)...

movieclip.alpha = 0;
object.addEventListener(MouseEvent.MOUSE_DOWN, dragObject);

function dragObject(evt:MouseEvent):void {
      object.startDrag();
      stage.addEventListener(MouseEvent.MOUSE_MOVE, checkObject);
      stage.addEventListener(MouseEvent.MOUSE_UP, endDrag);
}

function checkObject(evt:MouseEvent):void {
      if(object.hitTestObject(movieclip)){
          movieclip.alpha = 1;
      } else {
          movieclip.alpha = 0;
      }
}

function endDrag(evt:MouseEvent):void {
      stage.removeEventListener(MouseEvent.MOUSE_MOVE, checkObject);
      stage.removeEventListener(MouseEvent.MOUSE_UP, endDrag);
}