World Bank President A. W. (Tom) Clausen caused a sensation the other day merely by showing up in the bank's executive dining room for lunch. In all of his predecessor's 13 years at the bank, no one could remember Robert McNamara pubicly breaking bread with the lesser officials working for him.

"It's a new era," says a bank official who believes that Clausen -- the former chairman of the Bank of America -- intends his door to be open to the bank's staff. In recent years, according to bank insiders, staff morale suffered because McNamara kept the decision-making power tightly in his own hands, consulting outsiders more than the bank staff itself.

Clausen has been on the job only a short time. ("It's just 17 days," he reminded this interviewer at the end of last week.) But he doesn't need more time than that to know how suspicious the Reagan administration is of the multilateral lending approach typified by the bank. Congress, moreover, is pinching pennies, and one important key Reagan administration official, Treasury undresecretary Beryl Sprinkel, has publicly raised the question whether the bank has "socialist" leanings.

Clausen said in the interview that he has a major educational job to do in bringing things it perspective. He's impatient with "this whole litany of charges," including the accusation that the bank "jst throws money at problems," or that, "we are not supply-siders."

"This is just a wrong perception," Clausen said, "and we have to get the perception closer to what we're doing." He rejects, with a flat and precise "no," the idea that the bank is too big, a favorite theme of many ultra-conservatives. "Bigness is sometimes in the eye of the beholder," he says. But he is quick to admit that, measured by private institutions, the bank may not be a model of efficiency. He has been much impressed with the "talent" of the staff, the "quality" of its deliberations and "its sense of mission which I share." But he thinks there is room to tighten up on the bureaucracy in Washington, and in actual loan operations in the field.

"The first priority here," Clausen said, "is that the World Bank has got to be a bank. It's got to retain the confidence of its shareholders and of the marketplace. If it loses that, then we're down the tubes."

Is there any danger that confidence in the bank is being lost?

"You bet," Clausen says crisply. "The United States government is raising the question." Diplomatically, Clausen insists that the Reagan administration is being "supportive," although not as much as he would hope for in a "utopian world." His bigger complaint is that Congress for the past two years "has been kicking around" the $3.2 billion appropriation for the soft-loan affiliate, the International Development Agency. He suggests this is happening because politicians find no constituency for what they too casually label "foreign aid."

The Reagan administration's sniping at the bank is of course an outgrowth of the president's overwhelming commitment and belief in the virtues of the private sector. But Clausen, with impeccable free-enterprise credentials of his own, has a more practical view.

"You can't do it all by the private sector, and you can't do it all the bilateral way," he says firmly. "And the beauty of multilateral aid is that it cannot be politicized."

As he sees it, the challenge of the 1980s will be to find the right blend of bilateral, ultilateral and private-sector aid for poor countries, instead of allow a confrontation to develop among backers of the various kinds of help.

But Clausen's real problem -- and he knows it, although he is reluctant to talk about it too much -- is not so much the current IDA appropriation, but whether or not there will be a next one. The administration has under way a review of its relations with all multilateral agencies, but some officials have broadly hinted there may be no more money for IDA after this round.

"I've got to feel and probe my way in this," Clausen admits. "We've been doing a lot of thinking about that."

He feels he must be supportive of Reagan's efforts to reduce government spending, because a reduction of inflationary pressures would help everyone, including the Third World. At the same time, he senses that throughout the world, "the mood is moving against ever-increasing grants." If IDA is nearing the end of its existence, "a good manager will have to go to the drawing board and look at the alternatives. And I consider myself a good manager."

He doesn't expect to perform miracles -- and with it all intends to preseve his sense of humor, "and to have some fun." Like many of the top brass from the business world who slip into their first Washington assignment, he senses a certain loss of one kind of power (while gaining another).

"I could put a note in the out-box saying: 'Fix it.' I did that the other day, and I found a note in my in-box: 'You got to be kidding.'"