theres a cave that opens up and to get it open you have to stay underwater for 90 seconds, it has the stamp, a 1up & a lot of time, thats how you get it, some levels have tricky stamps and this is one of the worst ones

when there's the long clear pipe after the checkpoint go on top of it and then when the pipe goes up jump on it (you may need a cat suit) and you will see a stamp in the air but if you go through this pipe normally and the camera changes you will see shining lamp from the stamp

hope that helped :))