<svg xmlns="http://www.w3.org/2000/svg">
<clipPath id="clip1">
  <rect width="400" height="400"/>
</clipPath>

<g clip-path="url(#clip1)">
  <rect width="400" height="400" fill="red"/>
  <rect width="200" height="200" fill="green"/>
</g>

<script>
var clip = document.getElementById("clip1");
var rect = clip.firstChild.nextSibling;
rect.setAttribute("transform", "scale(0.5)");
</script>
</svg>