Mildred tells Montag that she thinks that a man called McClellan hit Clarisse in his a car and killed her. However, when Montag presses her, she says she is not entirely sure if Clarisse actually died. It is only when Montag asks his boss that he finds out for sure.
Beatty tells him that Clarisse's family had been on the government radar for a while due to what he calls their anti-social behavior. Normally he says that they "nip most of them (the ones as young as Clarisse) early," but it seems she was too far gone for them to be able to change her. He doesn't tell Montag how she died, but he does say that "the poor girl's better off dead."
Montag thinks he has in insight into how Clarisse dies when he is on the run and a car full of teenagers tries to mow him down. He wonders if they were the ones that tried to kill Clarisse. He is in no doubt that they are out to kill him because they turn around and try to hit him again.
Clarisse has a profound effect on Montag and is arguably the catalyst for his change. It is Clarisse that reminds him of the person he is and can be and opens his eyes to what is happening in the world. When she dies, Montag stops going into work and begins to seriously question everything that is happening around him.