A lot of things.

XF/Entity/Forum.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace RomhackPlaza\Master\XF\Entity;
+
+use XF\Mvc\Entity\Structure;
+
+class Forum extends XFCP_Forum
+{
+    public static function getStructure(Structure $structure)
+    {
+        $structure = parent::getStructure($structure);
+
+        $structure->relations['Club'] = [
+            'entity' => 'RomhackPlaza\Master:Club',
+            'type' => self::TO_ONE,
+            'conditions' => 'node_id',
+            'primary' => false
+        ];
+
+        return $structure;
+    }
+}