If the <def> block containing a <clippath> appears after any elements that use that clip path, they are not clipped.  There seems to be nothing in the svg spec which requires that the object refereed to appears above what refers to it.  The only place I could find anything about order mentioned was http://www.w3.org/TR/SVG/struct.html#DefsElement which only says that the <def> is 'encouraged' to be a direct child of an ancestor of the node that refers to it.

A simple example of this is (from https://github.com/ipython/ipython/issues/8133#issuecomment-93051239): 

<?xml version="1.0"?>
<svg width="120" height="120"
     viewPort="0 0 120 120" version="1.1"
     xmlns="http://www.w3.org/2000/svg">

    <rect x="10" y="10" width="100" height="100"
          clip-path="url(#myClip)"/>
    <defs>
        <clipPath id="myClip">
            <circle cx="30" cy="30" r="20"/>
            <circle cx="70" cy="70" r="20"/>
        </clipPath>
    </defs>


</svg>

or basically anything emitted by matplotlib's svg backend.

This bug has been reported to ubuntu (https://bugs.launchpad.net/ubuntu/+source/librsvg/+bug/1207538) and matplotlib (https://github.com/matplotlib/matplotlib/issues/4341, https://github.com/matplotlib/matplotlib/issues/4179, https://github.com/matplotlib/matplotlib/issues/2277) but apparently never made it upstream.