It’s another „Write it down so I don’t have to search for it on the internet all the time“-post!
– Rightclick on the object that should be the outer form of the Clip-Group and select „Set Clip Group“ (located near the bottom of the list). In the Layers and Object panel (Ctrl-Shift-L) the object will have been put in a group (that has a tiny scissor icon on its lower right) that contains another group called „Clip“ that contains the object.
– Now either click the „Clip“ group icon in Layers and Objects or double-click the original object on the canvas to get into the same state and draw a new object inside the Clip Group.
– You can also paste / move objects in Layers & Objects into the group, they have to be above the „Clip“ group that contains the original object
– With the original object in the Clip group selected you can change it’s shape (n), size (s) etc.

Thanks to this video by yjarz on Youtube linked by Polygon in this (German) thread I made about this very topic in the Inkscape forums.