How to access fields attached to taxonomy term in views-view-field.tpl.php?
Puh, finally I found a way.
This is my template file views-view-field.tpl.php (instead of a custom field I used the description field, but that doesn't matter in this case):

&lt;?php
if (isset($row-&gt;_field_data['nid']['entity']-&gt;field_myfiled['und'][0]['tid'])) {
  $tid = $row-&gt;_field_data['nid']['entity']-&gt;field_myfield['und'][0]['tid'];
  $term = taxonomy_term_load($tid);
  $output = $term-&gt;description_field['und'][0]['value'];
} else {
  $output = $output;
};
?&gt;

&lt;?php print $output; ?&gt;