Although it is never specified in the text, and although there is the possibility that Clarisse is alive with her family, who moved, it is accepted by Montag and others that Clarisse was run over by a car.
"Whole family moved out somewhere. But she's gone for good. I think she's dead."
"We couldn't be talking about the same girl."
"No. The same girl. McClellan. McClellan, run over by a car. Four days ago."
(Bradbury, Fahrenheit 451, Google Books)
Human life is of such little value in this society that no one but Montag seems to care that she has been killed; Clarisse mentions earlier that because people speed, and because people take great risks in order to feel, many of her friends from school have been run over. Later, as Montag is escaping, he is almost run down by a car full of kids, who are trying to run people down on purpose; he wonders if they are the same ones who killed Clarisse. The possibility of the government deliberately killing her to prevent her subversive ideas is unlikely; they were busy with the upcoming war.