A little into the game, you'll get a quest to find the Clown costume from a guy wrestling with an alligator. The parts to form the costume are all found beyond a nearby ramp, which you can jump over by using
. Interact with all the alligators there to find the parts, and return to the quest giver.
Once he joins the party, go left towards another alligator blocking the path, and you'd be prompted to switch to the Clown costume. You can now press
to sound the horn.
Just keep pressing
for 1,000 times for the achievement to pop. It should take around 5-10 minutes, depending on how fast you spam the button.