As far as I understand your question, you want to create a clipping mask in a shape of your black rounded rectangle that clips an existent white rectangle. It's not completely clear from the screenshot whether the player interface is vector, or is a part of the screenshot. If it isn't vector, you will have to draw the corresponding vector rounded rectangle over your screenshot.
Illustrator needs to know which of your objects is a mask and which is the object to be clipped, so it always requires the clipping mask to be closer to the front. If your black rounded rectangle is already a vector shape, and is below your white rectangle, copy it to the clipboard, select your white rectangle and use the
Edit > Paste to Front menu command or press
Cmd+F. This will paste the copy of the black rounded rectangle above your white rectangle keeping its original position.
Then to make a clipping mask:
- Select both white rectangle and the upper black rounded rectangle.
Object > Clipping Mask > Make or press
- The clipping black object will disappear hiding the "unneeded" part of the white rectangle.