Clarisse didn't fit into the conformist, mind-numbed society that she lived in. She stood out; she was different; she questioned things, had real conversations, wanted to know the why behind things. As Beatty states to Montag after she has disappeared, she was an "odd duck," and that odd ducks are not necessarily easy to get rid of.
All versions of the books are different, so a page number probably won't be accurate. But for quotes on how Clarisse was not like other kids, look at Montag's last conversation with Clarisse. She states,
"I'm anti-social, they say. I don't mix...I haven't any friends. That's supposed to prove I'm abnormal...I'm afraid of children my own age."
She also doesn't like going to the fun parks, killing things with her car, playing sports, and enjoys real conversations and asking questions. All of those behaviors are not like the kids that are her age.
Later, after Montag gets back from burning the old lady's house, he asks about what happened to Clarisse. Mildred tells him,
"I think she's gone...whole family moved out somewhere. But she's gone for good. I think she's dead...run over by a car."
It is never officially confirmed, just rumors floating around the neighborhood. It is rather sad and a bit mysterious.
Clarisse meets an unknown end, and all because she was a bit different, and different in their society was definitely dangerous to the government's power. I hope that helped; good luck!