WILLIAM FRENCH SMITH, President-elect Reagan's old friend and personal lawyer, is coming to Washington to be attorney general at the end of the Justice Department's most traumatic decade. Mr. Smith has a reputation as an able, honest and conservative lawyer. He will bring many views of the job with him. One of these deserves the most intense scrutiny by the Senate Judiciary Committee during his confirmation hearing: what is his concept of the job of the attorney general?

The recent history of the Department of Justice suggests that the question is vital. Does Mr. Smith see the department he will head as John Mitchell did: a political arm of the White House? Does he see it as Griffin Bell did: an almost-independent ministry of justice not only immune from political pressure but actually isolated from the rest of the administration of which it is a part? Or does Mr. Smith have yet another idea?

The department must have reached, in 1974, the lowest point of its long history. It was so discredited by the stories of what had gone on behind its closed doors that serious consideration was given on Capitol Hill to breaking it up. The proposal was to create an independent agency, outside the reach of the attorney general, to investigate and prosecute criminal cases. The rest of the department, with its job of handling the government's other legal problems and of advising the president, would have remained under a Cabinet officer's control.

Under Edward Levi, Mr. Bell and Benjamin Civiletti, the department has regained much of its reputation for honesty and fair play. But in the process, Messrs. Bell and Civiletti have come close to creating, by administrative practice, the kind of independent ministry of criminal justice Congress decided not to create by law. Those who run its criminal division now have almost unlimited freedom in deciding whom to investigate and how to prosecute. Nothing is cleared with or told to the White House, and precious little (as far as investigations go) is even discussed with the attorney general.

This arrangement has served the cause of justice well. Whether it has served President Carter well -- or will serve other presidents well -- is another question. It left Mr. Carter in the dark when the politically sensitive issues involving his brother Billy arose. And it took not only the president but also the attorney general out of the critical decisions in the Abscam investigation.

There are those -- we are among them -- who think it is a mistake for senior officials to isolate themselves so completely from politically explosive issues. Presidents need to be told when their associates or old friends are in trouble or have unsavory associates. Yet the Bell-Civiletti arrangement was created because something was needed to ward off the dangers -- so clearly demonstrated in the Watergate era -- of senior officials meddling in law-enforcement operations.

Somewhere along the line, Mr. Smith is going to have to decide how he will handle this question and what kind of relationships he will have with the president and his own subordinates. It is of particular importance that the policies of an attorney general on such matters be known early in an administration. That not only avoids misunderstandings, but also helps define the qualities on which those he chooses as his subordinates should be judged. If Mr. Smith is going to delegate as much authority as his two immediate predecessors did, those subordinates will have at least as much influence as he does on the quality of justice the Reagan administration provides.