An Arlington grand jury will investigate a cocaine possession charge against former Washington Redskins halfback Terry Metcalf, who waived a preliminary hearing in the case yesterday in Arlington General District Court. Metcalf pleaded guilty to a separate driving charge.

Metcalf, 34, was arrested March 26 after Arlington police stopped his car in the 3800 block of Lorcom Lane.

While searching Metcalf's car, officers found one-half gram of cocaine and a knife in a door pocket. A charge of possession of a concealed weapon was dropped yesterday.

Police spokesman Tom Bell said that Metcalf began yelling and refused to give his car registration to Officer Debbie Altman, who had stopped his car after radar indicated he was speeding.

Under the terms of a plea agreement, Judge Joseph Gwaltney reduced the charge of reckless driving to improper driving and imposed a $500 fine, of which $400 is suspended for one year. He also ordered Metcalf to attend eight hours of driver improvement classes sponsored by the state Department of Motor Vehicles.

Assistant Commonwealth's Attorney Mark Exley said the charges of disorderly conduct, a misdemeanor, and possession of cocaine, a felony, would both be heard by a Circuit Court grand jury "for the economy of having one court handle two charges on the same defendant." The grand jury is scheduled to meet July 21.

Metcalf's attorneys, Melanie Dorsey and Wallace Christensen, declined to comment on the case. Metcalf, who lives in Arlington, could not be reached.

Metcalf, whose arrest report said he was unemployed, was a star player for the St. Louis Cardinals. He retired before the 1982 National Football League's regular season after one year with the Redskins as a spot-player.