The NCAA has weekly polls available going back to 1999, and in that time Clemson became the first AP Preseason Number 1 to fall to fourth or worse while undefeated.
Edit: https://www.espn.com/college-football/s ... -no-3-poll says:
"The last preseason No. 1 to drop as far or farther without a loss or tie was Oklahoma in 1977. The Sooners slipped from first to fifth after opening the season with a 25-23 victory over Vanderbilt."
"One has to go all the way back to 1971 to find a similar drop to Clemson's for a preseason No. 1. Notre Dame started that season top ranked and won its first five games, slipping to No. 7 while doing so."