I can't empty the trash

Hi, I'm trying to empty the trash but the contents of a time machine backup in the folder won't delete. I've seen another thread on this site saying to not delete backups but I can't see how that can work here.

The file was originally a time machine backup from an older (no longer existent) time machine capsule. The file was held on an external drive but I connected the drive, tried to delete this file and some of it will not delete.

I can't delete it in safe mode either. I used to have been able to do secure delete but that's gone with high sierra so I'm out of ideas of how to clean the trash.

I don't think it a good idea to connect this partially deleted file to my current time capsule to then try and delete it in time capsule.

Thanks