<!DOCTYPE html>
<style>
#r {
    -webkit-clip-path: circle(50%);
    clip-path: url(#c);
}
</style>
<svg width="200" height="200">
  <clipPath id="c">
    <rect width="100" height="100"/>
  </clipPath>
  <rect id="r" width="200" height="200" fill="green"/>
</svg>