Prova questo > <?php
//if single post then add excerpt as meta description
if (is_single()) {
?>
<meta name="Description" content="<?php echo strip_tags(get_the_excerpt($post->ID)); ?>" />
<?php
//if homepage use standard meta description
} else if(is_home() || is_page()) {
?>
<meta name="Description" content="whatever your blog/site is about goes here w00t!">
<?php
//if category page, use category description as meta description
} else if(is_category()) {
?>
<meta name="Description" content="<?php echo strip_tags(category_description(get_category_by_slug(strtolower(get_the_category()))->term_id)); ?>" />
<?php } ?>
fonte http://www.kohactive.com/html/labs/wordpress-hacks-using-post-excerpts-as-meta-descriptions/ .