Ok, so I figure it would need to be like this ...
1. Select a small area of evenly-lit wall:
2. Ctrl J to put it on its own layer, then move it to a different part of the image, so it's visible:
3. Ctrl J it again, and move it across and line it up with the other one. Then Ctrl E to merge them together, then use the clone tool to meld the join if necessary. Keep repeating this over and over ...
... and over and over, until you have enough to cover the whole space:
4. Move it into place, and mask it on: