A prime suspect has been identified in the slaying last October of a young Time-Life Books employe who was shot in her newly rented town house in Alexandria's Old Town, police said yesterday.

Law enforcement officers refused to release details of any new development, but one individual familiar with the case said he understood a jail inmate in Pennsylvania had implicated himself in the crime in a conversation with a cellmate. The cellmate passed the information to authorities, the source said.

Deputy Alexandria Police Chief Arlen Justice said yesterday a break in the investigation of the slaying of the woman, 36-year-old Constance Mellon, occurred about two weeks ago when police received information from a source he declined to name.

Justice, who declined to discuss further details, said the case will be presented to an Alexandria grand jury scheduled to meet on Monday.

Alexandria chief prosecutor John E. Kloch would neither confirm nor deny yesterday that there were new developments in the case.

Mellon's death heightened anxieties among Old Town residents already made fearful by the slaying of a socially prominent young woman in her town house in 1978 and the fatal shooting of a woman shop owner in August of last year.

Another Time-Life employe, Mark Sawtelle, was seriously wounded in March 1980 during a robbery attempt near company offices on the city's main thoroughfare.

One city official said hundreds of persons in Alexandria and beyond have been interviewed by police and other law enforcement agencies in the 10 months since Mellon's bound body was discovered last Oct. 7 with a single gunshot wound in the head.

Herb Sorkin, vice president of Time-Life Books, said yesterday he did not know whether a $5,000 reward offered by the publishing company shortly after Mellon's death had figured significantly in the case.

Mellon, who was unmarried left a job at Random House in New York in September and moved to Alexandria to become a production executive at Time-Life. The company housed her at the Old Town Holiday Inn until she rented a narrow, two-story town house in early October a few blocks from her office.

Police said later Mellon had begun moving her belongings into the house on a Friday afternoon. A neighbor went to the door to complain about loud music from a radio sometime Saturday evening, but got no answer.

Mellon's body was discovered the following Tuesday after coworkers became concerned about her absence. She had been shot once in the head with a small caliber handgun in an upstairs bedroom, according to authorities.

An autopsy at the time reportedly showed no evidence of sexual assault.

Police said most of Mellon's possessions were left intact in packing boxes, but they declined to say whether an antique diamond ring, which friends said she wore, was missing.

Some of Mellon's coworkers theorized that she might have bcome careless about her personal safety after living for about 10 years in Manhattan. "You don't expect it [violent crime] when you live in a small, toy town," said one Time-Life employe at the time.