Hi, the roundabout is already mapped tagged https://www.openstreetmap.org/way/548130634
If you were wanting to map the boundary of the inside ring of concrete you could tag barrier=kerb/curb? 
https://wiki.openstreetmap.org/wiki/Tag:barrier%3Dkerb
Or if there was a garden on it you could map leisure=garden
Can you delete way you added or change the tagging thanks.