How can a field be rendered as a  element?
try to return drupal_add_html_head array directly

&lt;?php

function THEMENAME_field__field_test($vars) {
    return drupal_add_html_head(array(
        '#tag' =&gt; 'meta',
        '#attributes' =&gt; array(
            'content' =&gt; drupal_render($vars['items'][0]),
            'name' =&gt; 'test'
        )
    ), 'test');
 }