Clinton Portis could be headed to prison for a LONG time ... 'cause he's now facing up to 10 years behind bars after he agreed to plead guilty to a fraud charge last week.
According to court docs, obtained by TMZ Sports, the former NFL star cut a deal with prosecutors on Friday ... and pleaded guilty to one count of conspiracy to commit health care fraud.
As a result, per the docs, 40-year-old Portis now faces a maximum sentence of 10 years in prison .