What you do is first create a plane to slice with. The easiest way is to draw a lane that will go through the plane and then create a grounded work point at the angle and at a random distance from the line.  You can then use the two endpoints of the line and the workpoint to make the plane.  Now you can draw a sketch on the surface of the part that you want to remove (or on another plane beyond the surface that you want to remove).  On this sketch draw a rectangle larger than your part and cut-extrude it using the 'extrude to' feature and clickon the plane you created.