A muckraker spreads scandals, usually for political advantage. Being a muckraker is considered sleazy.
Muck can mean feces or dirt, and a rake could spread it around. Similarly, a muckraker spreads around something that's dirty in another way: news of scandals, real or fake. Muckrakers are kind of like gossips, but they're more public. Many politicians are muckrakers when they talk trash about their opponents. Newspapers can be muckrakers too, if they spread word of scandals, especially ones that are hard to prove. Muckraking means about the same as mudslinging.