Police say Clayborn left his vehicle, reached inside the taxi window and punched the driver before a friend pulled him away at about 2 a.m. on Jan. 18.
The police complaint said the cabbie sustained minor injuries.
Two months later, the 20-year-old Clayborn was charged with assault causing bodily injury, a serious misdemeanor. He turned himself into police on Friday morning. Iowa's athletic department says he pleaded not guilty.
If convicted, Clayborn could face a year in jail and a $1,875 fine.
"I've been aware of this alleged incident since January and consider it to be a serious situation," coach Kirk Ferentz said in a statement. "Based on what I know to this point, I think it is best to let due process run its course before coming to any judgment."
Clayborn, of St. Louis, started 11 games last season as a sophomore on a Hawkeyes squad that finished 9-4 and beat South Carolina 31-10 in the Outback Bowl.
Clayborn was a key member of one of the nation's top defensive lines in 2008, and he's expected to play an even larger role next season with the loss of star tackles Mitch King and Matt Kroul.
Iowa would return just one starting defensive lineman, junior Christian Ballard, if Clayborn is dismissed from the team.
The Hawkeyes, who should be among the favorites for the Big Ten title in 2009, start spring practice Wednesday. Ferentz is scheduled to hold a news conference that day.