$meta_boxes[] = array(
        'title'  => 'Multi-level nested groups',
        'fields' => array(
            array(
                'id'     => 'group4',
                'type'   => 'group',
                'clone'  => true,
                'fields' => array(
                    // Normal field (cloned)
                    array(
                        'name'  => 'Text',
                        'id'    => 'text4',
                        'type'  => 'text',
                        'clone' => true,
                    ),
                    // Nested group level 2
                    array(
                        'name'   => 'Sub group',
                        'id'     => 'group2',
                        'type'   => 'group',
                        'clone'  => true,
                        'fields' => array(
                            // Normal field (cloned)
                            array(
                                'name'  => 'Sub text',
                                'id'    => 'text4',
                                'type'  => 'text',
                                'clone' => true,
                            ),
                        ),
                    ),
                ),
            ),
        ),
    );