Controller.php

<?PHP
class MeuController extends AppController
{

    public function busca($txt){
        $resultados = $this->Model->find('all',array(
            'fields'=>array('*',"(MATCH(Model.title,Model.content,Model.campo_adicional) AGAINST ('{$txt}')) as relevancia"),
            'conditions'=>array('relevancia > 0'),
            'order'=>'relevancia DESC',
        ));
    }

}