That depends on whether or not you want A) movement in a random angle, or B) just want to bounce it around a random point on stage repeatedly...

Code:
// for A)
//...
var deg2rad:Number = Math.PI/180;
var speed:Number = 3; // set to speed you want

someClip.addEventListener(Event.ENTER_FRAME, moveIt);

someClip.rotation = Math.random() * 360;
var angle:Number = someClip.rotation * deg2rad;

function moveIt(e:Event) {
     e.target.x -= speed*Math.cos(angle);
     e.target.y -= speed*Math.sin(angle);
}
// for B)
// ...
someClip.addEventListener(Event.ENTER_FRAME, bounceIt);

function bounceIt(e:Event) {
     e.target.x = Math.random()*stage.stageWidth;
     e.target.y = Math.random()*stage.stageHeight;
}
Untested.