JUST WHEN YOU think Washington's prison mess might be clearing up and moving out of the headlines for at least an hour or two, somebody in the negotiations pulls a bureaucratic fast one and the emergency is back just as big as ever. Only last Friday, Mayor Barry shucked any remaining reluctance he may have had to agreeing on a specific in- city site for a new prison. That was important -- and it should have been enough for the Justice Department to ease its pressure on the city and resume relieving overcrowding by taking inmates in federal facilities. So is Justice agreeing to help out?

On the contrary, the Reagan administration now appears to be raising the ante: federal officials want a new commitment from the city to come up with yet another correctional facility or facilities -- trailers or a school or something temporary -- for some 400 more inmates above and beyond what the new prison and additional modular facilities at Lorton would accommodate when completed. So much for any good-faith understandings that local officials may have thought they had. As of yesterday, Justice Department officials wouldn't even agree to testify at a Senate hearing today on this and other local prison matters, claiming they needed more notice. What is the complete federal position, anyway?

Sen. Arlen Specter of Pennsylvania, who pressed successfully for federal assistance to get a new prison built in this city, may well wonder. The city's acceptance of federal help on this project is not and should not be an invitation to the administration to step on home rule and seize control of local criminal justice functions. This same administration tried to do this when it sought revisions in the home rule charter -- with the message that the people of the District and their government are somehow incapable of dealing with crime.

That is as wrong as it is offensive. The administration owes Congress as well as the public a clear understanding of its attitudes toward the city and its ability to help a burdened correctional system in a constructive manner.