Hi Ivo,
Thank you for posting here.
It doesn't support to set permission for one specific workitem. If you don't want others to see/edit some workitems, you need to make those workitems into one Area. For this Area, you could set if people can edit/view workitems in this node.

Best regards
We are trying to better understand customer views on social support experience, so your participation in this interview project would be greatly appreciated if you have time. Thanks for helping make community forums a great place.
Click
HERE to participate the survey.