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.
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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.
They still lead the Big Ten race, the winner of which will go to the Rose Bowl.