Draw for example a rectangle to your "small movie" ( using add visual elements-> paint->) Enable it's properties... Then add these lines in movie script(When movie starts script..)

// these 2 lines make a mask to your small movie
var mymask = element ("maski");
this.mask = mymask;

// test that masking is working...
this.graphics.beginFill(0xFF0000);
this.graphics.drawRect(0, 0, 800, 20); // draw very long rect..