I assume you are asking what really happened to Clarisse after Guy Montag knew her -- why she did not appear in the book after a while.

We do not actually know what happened to her.  However, it seems clear that she is dead.  Beatty seems to be telling Montag this at one point in the book.   That's the point where Beatty says that Clarisse is better off dead. We also see that Guy thinks she is dead when he wonders if the kids who are trying to run him down are the ones who killed Clarisse.