Cannot see your side view. It is called a stop cock.



Number 1 bolt is to tighten the lever. The second bolt can be taken out and replace the rubber washer that would solve your problem.

But the problem with most plumbing is, they embed the second screw inside the wall all the time. End up it is not serviceable. And hacking the tiles is the only answer.

Unless the plumber is patient enough to grind out the area around it to gain access to the nut, (Which needs grinding tool and lots of time). Just to replace the rubber washer.