Kdenlive/Manual/Project Menu/Clean Project
Clean Project
Available from the Project Menu this function removes any clips from the project tree that are not currently being used on the time-line. The files remain on the hard drive and are only removed from the project tree.
You can undo this action with Ctl-Z
This different from the Delete Unused Files button on the Project Files Tab in Project settings which deletes files not used by the project from the hard drive.