Hi @Attila Dob ,
Thank you for posting in this community.
No, it is not possible to manually clear out the retention hold library. The retention hold library is a hidden system location that is automatically created by SharePoint and OneDrive to store files that need to be retained for compliance reasons. It is not supported to edit, delete, or move these automatically retained files yourself.
Unless, Excluding your OneDrive for Business site collection from the Retention policy and post the removal, you can proceed for the deletion of content in the preservation hold library. Reference: How to Delete Files from the Preservation Hold Library in SharePoint Online? - SharePoint Diary
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