function wpspec_menu_desc( $item_output, $item, $depth, $args ) {
if ($item->description) {
$item_output = str_replace( '</a>', '<span class="description">' . $item->description . '</span></a>', $item_output );
}
return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'wpspec_menu_desc', 10, 4 );