Third-ranked Nittany Lions lost to Iowa Saturday night.
With 6 seconds left on the clock, Iowa kicked a field goal that put them ahead by one point, 24-23.
This game has left Penn State's hopes of a national championship in jeopardy.
Clark, who played a large role in the Lions' 9-0 start, struggled against the Hawkeyes.
The junior framed the game with a near-fumble on Penn State's first drive and a costly interception on its last possession.
There was definitely plenty of blame to go around.
"I want to apologize to the whole Penn State nation for my play today," said Clark, who completed just 9 of 23 passes for 86 yards, no touchdowns, and the one interception. "I take full responsibility for that loss."
The Lions, 9-1 overall and 5-1 in the Big Ten Conference, will have to regroup for their final two games.
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They still lead the Big Ten race, the winner of which will go to the Rose Bowl.