public Polygon GetClipGeometry()

{
    var mapWithClip = MapView.Active.Map;
    //If clipping is set to ArcGIS.Core.CIM.ClippingMode.None or ArcGIS.Core.CIM.ClippingMode.MapSeries null is returned
    //If clipping is set to ArcGIS.Core.CIM.ClippingMode.MapExtent the ArcGIS.Core.CIM.CIMMap.CustomFullExtent is returned.
    //Otherwise, if clipping is set to ArcGIS.Core.CIM.ClippingMode.CustomShape the custom clip polygon is returned.
    //Note: Must be on the QueuedTask.Run()
    var poly = mapWithClip.GetClipGeometry();
    //You can use the polygon returned
    //For example: We make a polygon graphic element and add it to a Graphics Layer.
    var gl = mapWithClip.GetLayersAsFlattenedList().OfType<GraphicsLayer>().FirstOrDefault();
    if (gl == null) return;
    var polygonSymbol = SymbolFactory.Instance.ConstructPolygonSymbol(CIMColor.CreateRGBColor(255, 255, 0));
    var cimGraphicElement = new CIMPolygonGraphic
    {
        Polygon = poly,
        Symbol = polygonSymbol.MakeSymbolReference()
    };
    gl.AddElement(cimGraphicElement);
}