<?php
    $id=$item->id;
    $db = JFactory::getDbo();
    $query = 'select * from #__fields_values where item_id = "'.$id.'"';
    $db->setQuery($query);
    $fields = $db->loadObjectList();


    foreach ($fields as $field) {
        if ($field->field_id == 3) { // 4 - id of your custom field
        echo $field->value;
        }
    }
    
    ?>

Pour en savoir plus :

https://docs.joomla.org/Selecting_data_using_JDatabase/fr

Ou bien

https://joomla.stackexchange.com/questions/23728/custom-fields-inside-module-override