Although stamps can be found in really tricky places, and sometimes we don't even realize they are there, and we pass them without even realizing it, in world 6-1, Clear Pipe Cruise, you will find it if you travel through the pipes in a place in particular, so you will find the location of the stamp, you must also have the catsuit activated because it becomes easier to reach at the beginning of the level, because there are no places to obtain it during the tour.
At the checkpoint flag you will have a pipe ahead so that you can reach the end jump and move along with it and when it turns up on the pipe wall, and you will see the stamp floating, if you do not have the catsuit you will need a turn jump almost perfect for getting the stamp, so it's easier to climb the pipe to stay alive.
We hope this article helps you, and you can see in the Super Mario 3D World + Bowser’s Fury game where to find the stamp in the World 6-1, in the easiest and most fun way.