virtual_field.php

<?php

function mytheme_alpha_preprocess_node(&$vars) {
    $node = $vars['node'];

    if (isset($vars['field_topic'][0]['tid'])) {
        $parents        = taxonomy_get_parents($vars['field_topic'][0]['tid']);
        $top_parent     = array_shift($parents);
        $vars['topic']  = field_view_value('node', $node, 'field_topic', array(
                            'tid' => $top_parent->tid, 'label' => 'hidden'));

        $vars['topic_rendered'] = theme('field', array('element' => array (
            '#title'    => $vars['content']['field_topic']['#title'],
            '#field_name'       => 'field_topic',                        /* imitate source name too */
            '#field_type'       => $vars['content']['field_topic']['#field_type'],
            '#entity_type'      => $vars['content']['field_topic']['#entity_type'],
            '#bundle'           => $vars['content']['field_topic']['#bundle'],
            '#label_display'    => 'hidden',
            '#items' => array('0' => array(
                                        'tid' => $top_parent->tid,
                                        'taxonomy_term' => &$top_parent
             )),
            '0' => &$vars['topic'],
        )));
        unset($parents, $top_parent);
    }
    else {
        $vars['topic'] = array();
        $vars['topic_rendered'] = array();
    }

}

?>