Clarisse is talking to Montag about how fast cars whiz by when she first mentions her uncle getting arrested. This takes place during Clarisse's and Montag's first meeting in the first handful of pages right at the beginning of the book. She says that her uncle wanted to see what things looked like when he drove past them, so he slowed down to forty miles per hour, which got him locked up in jail for two days.
About a page later, Clarisse mentions that her uncle was arrested again, this time for "being a pedestrian." Interestingly enough, Clarisse offers this information willingly and almost proudly. Most people might have an ounce or two of shame about their family members being arrested, but Clarisse knows enough about how backward their world is not to care in the slightest. This sort of cavalier attitude about the law almost certainly lays the foundation for Montag's eventual abandonment of their society's rules.
Later on, when Captain Beatty comes to visit Montag in his house, he says that the firemen have kept a close eye on Clarisse and her family. Although he says the uncle has "had a mixed record," Beatty declares that they never found any of them in possession of any books.
Clarisse tells Guy in The Hearth and the Salamander that her uncle has been arrested before for being a pedestrian. She is describing to Guy why her family is so peculiar because they sit around and talk and they go for walks for no reason. This is unusual because talking and walking and observing and thinking are all in stark contrast to the people in this society. People who are against the norms in society in any way are arrested.