This is not a real solution but the best approximation I could find.
The shape acts like a clipping one only when you enable some animation.
- Select the shape with your overflowing text
-
Format>Text, Text Animation tab
- Set Effect to
Scroll in and eventually tune the other parameters
After the progressive scroll in, only the central part of the text remains visible inside the shape.
Apparently, you can’t align the text left or right; it is centered only.
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