Looks like 7mm to me.

Still, I think it is fine and I would ride that without worry. A new front tire is in your future anyway (this one looks worn, but you could probably move it to the rear). There might be a tendency for debris to get into the little cut, but I wouldn't worry too much about it.

Thinking more about this, if your rear tire is even more worn than the front, get two new tires. Don't put a worn tire on the front, or leave a cut tire on the front. Failure of the front tire is more problematic.