Black's sacrifice at is the key to making an eye at the top edge in sente. Black's four in a row on the second line is good enough for an eye, and he can make the second with in the space in the middle (a and b are miai).
The clamp at is wrong. White can connect at , and to secure the eye on the top, Black must connect at , this time in gote. After , there is only one eye at the top. See six die but eight live for more information.