If arc have a 3d value it's should be in 3d polyline. Mapclean function only changes arc entity type as polyline. For converting arc to polyline i copied all arcs in new layer. Because I think its will not convert direct to polyline or 3d polyline.
Select the arc, right click and select "Convert to Polyline". Warning it does seem like it explodes the arc and creates a lot of lines and then connects them as a polyline.
