I would start by dragging two guidelines though the horizontal and vertical center of the circle.Then using the Shape Editor Tool (set to Line not curve) draw from the center a slice triangle. Click the last control point on top of the first to create a closed shape (it will fill black). Select both the slice shape and the circle and press Ctrl + 4 or Arrange > Combine Shapes > Slice Shapes.

This will maintain the circle but create a new shape for the slice.