Something

this.createEmptyMovieClip("circle_mc", 1);
with (circle_mc) {
	beginFill(0x000000, 0);
     lineStyle();
     lineTo(100, 0);
    lineTo(100, 100);
      lineTo(0, 100);
     endFill();
     onEnterFrame = function(){
        this._alpha+=10;
if(this._alpha >= 100){delete onEnterFrame;
this._alpha = 100;
}
}