When Clarence M. Kelley talks about his four embattled years as FBI director, one can almost hear in the background the bittersweet words of "September Song" with their insistent reminder of how "the days dwindle down to a precious few."

For Kelley, that's all that remains. He may still occupy the office that once served the late J. Edgar Hoover as an instrument of awesome personal power. But his presence there is an anomaly.

In the political and bureaucratic worlds of Washington, no one fears, or even thinks very much about, a "lame duck." Yet, a "lame duck" is what Kelly has been since he announced in January that he plans to leave the FBI by year's end.

For a man who has given 25 of his 65 years to a love affair with the life and traditions of what he reverently calls "the bureau." It wasn't an easy decision. But Kelley had his reasons.

The job was chipping at his health. President Carter and Attorney General Griffin B. Bell had signaled politely but unmistakably that they would prefer the FBI to be run by someone of their choosing. Most important, Kelley's intimates say, he simply decided that, for better or worse, he had done all that he could to resolve the problems of the FBI.

And so official Washington already has closed its books on the Kelley years and turned its attention to speculation about his successor. For Kelley, there seems little to do but watch "the days dwindle down" toward the pension for which he will become eligible Oct. 1.

Still, Kelley is determined that these final months aren't going to be just a waiting game. That he's still the boss was demonstrated a few days ago as Kelley sat at his desk almost around the clock, quietly but authoritatively acting as the federal government's "point man" in events being watched all over the world.

First, there was a gunman in Ohio waving a pistol at a hostage and demanding to talk with the President. Then, literally within minutes of the Ohio hostage's rescue, came "the siege of Washington," with its agonizing spectacle of a handful of desperate men holding more than 100 hostages.

In both instances, it was Kelley who commanded the substantial federal police presence assembled behind the scenes to advise and, if necessary, take over from local officials. For most of a week, he hardly moved from the space-age telephone console by the side of his desk, issuing instructions and asking questions of his agents on the scene and relaying hourly updates of the situation over his direct line to Bell in the Justice Department.

Perhaps the most revealing thing about the FBI's role in these events was that the public was hardly aware of it. That wouldn't have been the case had the incidents happened in Hoover's day. Hoover's juggernaut public-relations machinery would have moved in to elbow the local police aside and snatch as much of the credit as possible for the FBI and its director.

By contrast, FBI officials say, Kelley insisted in the Ohio and Washington cases that the FBI stay in the background and assist but not interfere with local authorities. He scrupulously followed the same rule when the laurels were being handed out for the successful release of the hostages.

It was a telling illustration of what Kelley calls the "non-swashbuckling style of police administration" that he has followed throughout his four years as FBI director.

To his critics, that style lies at the root of what they consider as Kelley's failure to restore the FBI's tattered, scandal-torn image. But Kelley remains convinced that when the final balance sheet on his performance is toted up, it will prove to have been his most significant contribution to the bureau.

There's no question that Kelley's plodding, almost anti-heroic style has been a great drawback to his efforts to convince the public that the FBI isn't still being run from beyond the grave by the spirit of Hoover.

When he died in 1972 after 48 years as director, Hoover left the bureau a strangely mixed legacy.

First, through a combination of genuine achievement and skillful public relations, he built the FBI into the world's most famous police force. Then, as he slipped into old age and began to fear the loss of his power, he came close to destroying the FBI legend through reckless and often lawless vendettas against those whom he saw as his enemies.

When Kelley, a retired FBI agent who had served 12 successful years as police chief of Kansas City, was called to Washington in 1973, he found himself inheriting an organization under siege.

The scandals and abuses of the final Hoover years were gushing out of the FBI's secret files into congressional hearings and newspaper headlines. Internally, the bureau was dominated by men who had been raised in the Hoover tradition and who remained fiercely loyal to his memory and ideas.

Kelley's critics have always contended that he made a major mistake in not moving immediately to purge the FBI of the Hooverite old guard. As one law enforcement official says privately: "He should have demanded the keys, unlocked all the closets and thrown all the skeletons out on the public street. Then he should have called all the executives in and said, 'Boys, I want your resignations.'"

"The people who say that are people who think I was too naive or too honest for this job," Kelley says in reply. "Maybe they're right. But I've never thought so."

It wouldn't have been "fair," he adds. It's a word he uses a lot; and people within the FBI often say that it's characteristic of the way in which he seemed to approach most bureau problems - a constant musing aloud and asking subordinates about whether a given action would be "fair."

Instead, Kelley elected to bring reform to the FBI through gradual evolutionary means. Most of the old guard, he insists, "were willing to except change"; and he gambled out using them as a transitional force that would keep the FBI's wheels turning while helping him to train a new post-Hoover generation of leadership.

The problem, Kelley now concedes, is that he miscalculated the degree to which the past would prevent him from concentrating on the future. Throughout most of his four years, Kelleys says ruefully, "the press and public seemed overwhelmed by stories of the old abuses and underwhelmed by the changes we were making."

He quickly found that the problem wasn't to be solved through earnest assertions that all the bureau's misdeeds had been ferreted out and put on the public record. Each time Kelley made such a statement, some new horror story came tumbling out of the closet.

Eventually, these disclosures engulfed the FBI in two still-continuing investigations by the Justice Department: one involving allegations that FBI executives had misused bureau funds and property, the other focusing on illegal burglaries committed by FBI agents in the course of domestic security investigations.

The first of these probes forced Kelley to fire, without explanation, the man he had chosen to be the FBI's deputy director, Nicholas P. Callahan. The other led to the acute embarrassment of Kelley having to confess during a television appearance that his subordinates had deceived him about the burglaries.

Then, last September, Kelley became the target of charges that he had improperly accepted gifts and services from FBI personnel. He was exonerated by former Attorney General Edward H. Levi, but that didn't stop Jimmy carter, then campaigning for the presidency, from telling a cheering audience that he thought Kelley should be fired.

The net effect of these incidents was to create an impression of Kelley as a man out of his depth at the FBI and unable to cope with its internal intrigues. Undoubtedly, this impression has led to Kelley's unspoken acknowledgement that his usefulness to the bureau is ended.

But there is another side to the Kelley story - one that has generally escaped the notice of the headlines. Kelley thinks that with the perspective of time, though, it will come to be better understood and appreciated.

Although his gamble on a gradual approach to reform didn't enhance his reputation, he believes that its long-run effects will be of great benefit to the bureau.

During his four years, at a level not readily apparent to the public, the FBI has become an organization very different from what it used to be.

The serious problem of the bureau's accountability for its actions has moved a considerable way toward solution. Where Hoover scted as a law unto himself, ignoring the Judtice Department and even the White House, Kelley has acknowledged the need for higher supervision over the FBI and has cooperated in setting up the guidelines and review procedures to make such supervision work.

Under his prodding, the FBI has shifted its targets and methods from the simple, easily solved crimes favored in Hoover's time to the more sophisticated and challenging problems of organized crime and white-collar crime.

Perhaps most significantly, when Kelley leaves at year's end, the workings of a new retirement law will require between 555 and 650 senior agents to go too. That means that more than half the bureau's force of approximately 8,000 agent will be younger than 35.

From that time on, the controversies about whether the Hoover spirit still endures within the FBI should be largely ended. A new generation of younger FBI men, identified and schooled in special management programs set up by Kelley, will be moving into the leadership positions.

These changes are still in their embryonic stage and require considerably more work to ensure that their promise is realized. But, as Griffin Bell says:

"My impression is that 95 per cent or more of what the FBI does is done very well, even though for a long time they've been taking a lot of heat over things that happened some years ago and that actually involve only a small part of their work."

As for Kelley, it doesn't disturb him that someone else might reap the benefit of his reforms. "That's something that any administrator faces sooner or later," he observes. "It's often the case that man who makes the changes is not the man who will put them to work. If I've done anything for the bureau, it's that I hope I've made it capable of accepting change."