There are a few ways to crop just one layer in Photoshop. One way is to use the Selection tool, which is located at the bottom of the Tools palette. You can select a part of the image and then drag it up or down to crop that area. Another way is to use the Marquee tool, which is located at the top of the Tools palette. You can select an area and then press Command-M (Mac) or Control-M (PC) to marquee that area.
