Stamps in Super Mario 3D World + Bowser’s Fury can be placed in really tricky spots. Most of the time, you will venture right past them without any clue of them being there. In World 6-1, Clear Pipe Cruise, you are expected to travel through various pipes to collect items and get to the end. However, you need to go against this thinking at one particular spot to find the stamp here. Here is the location of the stamp in World 6-1 in Super Mario 3D World.
Before you start this level, this stamp is easiest to grab with a Catsuit power-up on. There are no spots to get it in the course itself, so we recommend going into it with one at least in your inventory.
Proceed through the level until you get to the checkpoint flag. Ahead of you will be another pipe for you to continue towards the end. You need to jump on top of this pipe and run along with it until it turns upwards. Climb the pipe wall, and the stamp is floating up here.
You can go through the pipe if you want, but you will be running back towards the camera. It is possible to get to the upper portion of the tube without the Catsuit, but it requires a near-perfect spin jump to pull off. To save your lives, it is simply easier to climb the pipe.