getChildByName.

for(var i:uint = 0; i<5; i++){
    this["mc"+i].addEventListener(MouseEvent.CLICK, clickHandler);
}

function clickHandler (e:MouseEvent):void{
     var n:int = e.target.name.substr(2);
     var mc:MovieClip = this.getChildByName("mc" + n) as MovieClip;

     mc.alpha = 0;
}


: e.target.alpha = 0; <- 이걸로 해도 되긴된다..- -;