Custom field as meta description

If you want to fetch the data from a custom field named wpse_desc:

then here’s one way to do that with your code snippet:

function wpse_custom_meta_description() 
{
    // Nothing to do
    if ( ! is_single() )
        return;

    // Fetch our custom field value
    $desc = get_post_meta( get_queried_object_id(), 'wpse_desc', true );

    // Output
    if( ! empty( $desc ) )
        printf( 
            '<meta name="description" content="%s" />',
            esc_attr( trim( $desc ) )
        );
}
add_action( 'wp_head', 'wpse_custom_meta_description' , 2 );

It’s important to escape the value of the content attribute, because you don’t want e.g. authors of your site to be able to write something like:

><script>alert('Hello World!');</script><

We might also want to strip the tags out with wp_strip_all_tags() and limit the word count with wp_trim_words(), that also uses wp_strip_all_tags(), but we should keep in mind that it would still allow very long words. If you need a maximum length, then you might need to look further into that, I’m sure there are various approaches out there.

Here we prefix the custom field name with the wpse_ prefix, to avoid possible name collisions.

Hope you can adjust it to your needs, like using post excerpt or post content fallbacks.