Movie clip question

[passworder]

I have a movie clip which contains a rectangle. I want to add another movieclip inside this rectangle, so I get a scrolling background inside this rectangular space. The problem is that if I create a scrolling background as it is, the rectangle movieclip becomes larger.

The question is quite hard to formulate, I can make an image to show what I mean if you don't understand what I meant.
Thankful for help..