Hi Karsten,
> is it possible to clone a Hosted Feature Service in Portal for ArcGis and give the "new" one different permissions (e.g. not editable) then the original service?
This is not a supported workflow in Portal for ArcGIS at this time. Workaround: I suppose you could re-publish the original source data as a new, separate hosted feature service (immediately after you publish the original one) with a different name to support this workflow.
Hope this helps,