I think your problem is that it is not inside the CDATA statement. Try this:
<description>
<![CDATA[<?php if($image = $item->coverimage()->toFile()): ?> <figure> <a href="<?php echo $item->url() ?>"> <img src="<?php echo $image->url() ?>"> </a> </figure> <?php endif ?><?php echo $item->{$textfield}()->kirbytext() ?>]]></description>
I also amended the link target, since you probably want that to go to the page the image is on, rather then the image directly.
