A lot of things.

app/Http/Controllers/WebhookController.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Models\Entry;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Cache;
+use Illuminate\Support\Facades\Log;
+
+class WebhookController extends Controller
+{
+
+    public function XenForoNewPost(Request $request)
+    {
+
+        if( $request->header('XF-Webhook-Secret') !== env('WEBHOOK_SECRET') )
+            abort(403);
+
+        $threadId = $request->input('data.thread_id');
+        if( $threadId ){
+            $entry = Entry::where('comments_thread_id', $threadId)->first();
+            if( $entry ){
+                Cache::forget("entry_comments_{$entry->id}");
+            }
+        }
+
+        return response()->json(['success' => true]);
+    }
+}