It's a fixed beam that's designed to bend in cornering. Loads of Clios seem to have slightly bent rear beams from minor low speed crashes (sliding into a curb at 3 mph on ice for example).
- Doesn't sound like tyre pressures (underinflated wears both edges, overinflated wears only the middle) but check them anyway as it's free.
- Jack the car up and check the wheel bearings don't have any play (just try to wobble the wheel). I don't know if it's applicable on the drum brake models, but the RS has very delicate rear bearings that need to be torqued and installed very carefully.
- If you're still at a loss, pay £30 to have the tracking checked & a print out. Then spend £30 or so on angled shims that go between the beam & hub (the idea being if toe is 1 degree out, you buy a shim with an angle of 1 degree and install it "backwards" to remove 1 degree of toe)