/**
 * Implements hook_field_attach_submit().
 */
function {{ machine_name }}_field_attach_submit($entity_type, $entity, $form, &$form_state) {
  // Sample case of an 'Empty the field' checkbox added on the form, allowing
  // a given field to be emptied.
  $values = drupal_array_get_nested_value($form_state['values'], $form['#parents']);
  if (!empty($values['empty_field_foo'])) {
    unset($entity->field_foo);
  }
}