It was quite hard to achieve before: corner of a cube.
1. PartDesign New part, New Body
2. Additive box. Just click OK.
3. Datum plane. Use 'Plane by 3 points' attachment mode. The plane should slice off the wanted piece from the cube.
4. Create a sketch on the plane, draw a big rectangle...
5. ...and Pocket away the unneeded part of Box. Done