It's difficult to tell from your screen shot, but I suspect there might be a transparency mask, rather than a clipping mask. In any case, I think transparency masks are the way to go to make something like this.
Here's an example below, showing a transparency mask applied to a group. You can see if you expand your layers, you should be able to find a group which is underlined. If you select it, and open up the Transparency window, you should be able to see if there is a mask applied.
You can then edit the mask by clicking on the mask thumbnail in the Transparency window. To switch out of editing of the mask, select the group thumbnail in the Transparency window.
You can then copy and paste an image as the background texture and send it to the back of the layer stack, so that the mask cuts out the group revealing the background image underneath.