Accessing Elements on the Stage from a linked Class

I’m trying to link a clip to a class, and it goes well until I try to access MovieClips or TextFields placed on the stage within that clip from code in the class. My code is below; there is a TextField called output_txt on the stage within this clip.

Any help is much appreciated.

class Main extends MovieClip{
public function Main() {
//trace(“123”); <–if I replace the line below with this, it traces nicely
output_txt.text = “123”;