Alright, so to get this achievement, you need to progress to nearly the end of the story.
You'll unlock it on your way to deal with Liberty and not by purchasing it from Masato and eating it.
So, as you reach Yuma county, one of your task will be to break the alliance between the Payaso and Godfishers and have them go to war. To do that, you'll need to travel to the Godfishers' windmill.
Once there, follow the way until you reach what's left of a building and there, you will be greeted by Risky Briskett, she'll ask if you've had a Clown Burger, so pick the option that let's on have her give you one. Once you eat it, the achievement will pop!
Hope it helps.