On second thought, I followed Moini's tag line advice and looked at the status bar. On Ctrl-A, it does say
2 objects selected of types Circle, Clone in root. ...
So it's not that Inkscape didn't read the <use> element. But why can't I see the clone? It's probably not outside of the viewBox, as I verified by widening it to "-20 -20 200 100". It's not simply below the circle: If I select that and drag it away (as described at
https://designbundles.net/design-school ... n-inkscape) nothing shows up underneath. But when I then Select All, it again selects both, and on View→Zoom→Selection, it becomes obvious that there's nothing at any other position. Maybe the clone is so tightly connected to the original that there's no way to move one without moving the other. My question then becomes: How can I sever that link? Is there a way to do that for all clones in a file at once?