It is not possible to split a line or polygon feature in ArcGIS Online Map Viewer. However, it is possible to do so using ArcGIS Web AppBuilder in ArcGIS Online, and the result is reflected at the web map level in Map Viewer. The following steps describe how to split polygon features and line features in ArcGIS Web AppBuilder.
Note: The following workflow is only applicable to web maps with editable hosted feature layers. Refer to ArcGIS Online: Manage editor settings for more information.
Note: Alternatively, on the Widget tab, scroll to and click Widget 1, 2, 3, 4 or 5 to add a new widget.
The images below show the cut line and polygon features in ArcGIS Web AppBuilder, with the result reflected at the web map level in ArcGIS Online Map Viewer.
In ArcGIS Web AppBuilder:
In ArcGIS Online Map Viewer: