#validate not overriding the submit function
Like Clive said resetting $form_state['input'] is the solution, however I'd rather handle both actions in the submit function:

function mymodule_myform_submit($form, &amp;$form_state) {

  if($form_state['clicked_button']['#value'] == $form['clear']['#value']) {
    $form_state['input'] = array();
    $form_state['rebuild'] = TRUE;
  } else {
    $values = $form_state['values'];
    die('the value you submitted: '.$values['title']);
  }

}