There are a few ways to achieve this ,but i would use the Multi Cut Tool on the cylinder in an orthographic view. Select the multi cut tool from modeling toolkit ,hold down shift and click to the left of your cylinder then move to the right of the cylinder and complete the cut by clicking when at the angle you need. Note that holding shift allows incremental degrees so you can get 45 degrees accurately.
Having made the cut select the faces above the cut and delete.To fill the hole select the top edge loop by double clicking on the top edge the either use Fill hole (under the Mesh tab ) and fix the ngon using the split polygon tool or after selecting the edge loop use the extrude tool by clicking the little blue circle to center and extract using the center box and then merge the vertices at the center.
Hope this helps.