The first time we hear of what happened to Clarisse is when Mildred tells Montag "Whole family moved out somewhere. But she's gone for good. I think she's dead." (pg 47) She goes on to tell him that Clarisse was run over by a car four days ago. She wasn't sure,but she was pretty sure.
Later, when Beatty is talking to Montag he says that Clarisse was a time bomb. Her family had been feeding her subconscious and she kept asking questions. That could lead to unhappiness. So, "she is better off dead".(pg 60) He says that luckily people like Clarisse do not appear often but that the government now knows how to "nip most of them in the bud.." (pg 60)
Clarisse has been "nipped in the bud" as they say and is dead.
The pages I have sited are in my edition of the book, but they should be somewhere close to that page.
In Fahrenheit 451, Clarisse dies, but Bradbury does not describe this event in the text. Mildred informs the reader of Clarisse's death. In Part Two, when Montag notes that he has not seen Clarisse for four days, Mildred declares that she is "gone" and that she was "run over by a car." Clarisse's family has also moved out and left the area. When Montag asks why she did not tell him sooner, Mildred says she "forgot."
Clarisse's cause of death is confirmed in Part Three. When Montag is running away from the Hound, he is almost knocked down by some joyriding teenagers. This prompts him to wonder if they were responsible for killing Clarisse, but this notion is neither confirmed nor denied by Bradbury.