First thing, Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar.
To remove unused clips...
Select all clips under the desired tab in the bin, using <ctrl-A>. (Not just the ones that are to be deleted.)
Right-click on one of the selected clips.
Check the "Do the same for all items" box.
Press the CANCEL button. (NOT the Delete button.)