Hi Thomas,
Thanks for using Syncfusion products.
It is not possible now, But we can achieve this by using the context Menu, it contains an item "Reset Sort", it'll be enabled when you right click on the header cell. It will reset the sort in the visible column.
Find the work around sample in the following location.
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=GDCSample-2048055786.zipPlease let us know if you need any more details.
Thanks
Ganesan