I assume you mean the Clone Stamp Tool, to rotate a selection with the rectangular marque tool it has to be on a separtate layer, so you have to make a select with that tool and hit CTRL J to copy it to a new layer then you can hit CRTL T to free transfer it
I'm a little confused about this bit "I am using the stamp tool and need to go up to a straight edge (i.e a picture frame)." and the title "Create a mask with a straight but rotated edge"
Can you explain a bit more about what you're trying to acheive, perhaps screenshots would help
Yes we understood. I would use the Polygonal Lasso Tool or the Pen tool to create a selection (marquis) to work within.
Here's an image where I want to remove the cup spill from the post-it note.
I use the Polygonal Lasso Tool (or the Pen Tool) to create a selection/marquee on the straight edges to serve as a mask (old airbrushing term).
Then I use the Clone Stamp Tool to clone out the spill........the mask (selection/marquee) preserves the straight edges.
Worked perfectly!! The downside is I felt on the stupid side since I have been using Photoshop enough years to have thought of the Polygon Lasso -- guess Lightroom does so much that I use to do in Photoshop that I have forgotten some of the "tricks".