An Alexandria grand jury yesterday indicted a Pittsburg jail inmate on murder and robberycharges in the Old Town slaying last October of Time-Life Books employe Constance Mellon.

The four-count indictment accused Frank Weston, 27, of premeditated murder, robbery, use of a pistol while committing murder and robbery, and being armed with a deadly weapon while breaking and entering.

The maximum penalty on the murder charge is death. If convicted, Weston could also face two life prison terms on the robbery and breaking and entering counts and a mandatory one-year sentence on the weapons charge.

The indictment gave the first public indication in the 10-month history of the case that money was taken from the newly rented town house where the 36-year-old Mellon was slain. The amount was not specified and Alexandria chief prosecutor John E. Kloch yesterday refused to elaborate.

Weston is in custody in the Allegheny County, Pa., jail, where he is awaiting a hearing on whether he will be given the death penalty for his conviction in the Oct. 4, 1980, slaying of a Pittsburg shoe store clerk.

Mellon's bound body, shot oncce through the head, was discovered in her second-floor bedroom last Oct. 7.

Kloch said Weston's return to Virginia to face the charges would be sought "espeditiously," but said Alexandria authorities would not interfere with legal proceedings in Pittsburgh.