Comment on WordPress SEO Meta Tags 2016 by SEO Dave.
I’ve looked into using the WordPress Tag description code for the Meta Description Tag and unfortunately the tag description code can’t be used within the meta description tag because it takes the text you add to the Tag Description box and wraps it in paragraph tags and you you shouldn’t use HTML within a meta description as it will be listed as text in Google.
Both of these if added to the appropriate part of the header.php file will use the Tag Description, but with paragraph code:
<?php echo term_description(); ?>
<?php echo tag_description(); ?>