A certain desperate moderation is in the ascendancy in the Arab world, post-Lebanon. The trick for Ronald Reagan, when he meets with an Arab League delegation here today, will be to nurture it without nurturing false hopes or hot rhetoric. The encounter cries out for the Trollope ploy.

That's the maneuver that seizes upon the most constructive of the discernible diplomatic signals, while seemingly ignoring the least positive. It has its literary antecedents in the novels of Anthony Trollope, and the messages his heroines read into under-the-table hand-squeezing. Its most celebrated diplomatic application came in the 1962 Cuban missile crisis, when President Kennedy chose to act on much the more constructive of two communications from Nikita Khrushchev.

As Ronald Reagan sets out to advance his "fresh start" on the "autonomy" talks and the Camp David peace process for settling the Palestinian issue, he too is confronted with a cacophony of conflicting signals. On the floor of the Knesset, Israeli Prime Minister Menachem Begin and his associates are given to categorical declarations: Judea and Samaria (the West Bank) are to be forever a part of Israel. Elsewhere, high Israeli officials take a somewhat more flexible view.

At their summit meeting at Fez, the Arabs offered teasing hints of an awareness of Israel's existence, while insisting on an independent Palestinian state as the only acceptable outcome, and Yasser Arafat as the only legitimate spokesman for the Palestinians. But Arafat is talking to Jordan's King Hussein. And Hussein, given his chronically precarious position, is showing an active interest in joining the "peace process."

Others among the moderates (Egypt, Saudi Arabia, Morocco) are forcing the pace. Never mind what this says about their deep-down devotion to the Palestinian cause. While that issue lies smoldering, it is a threat to their stability -- the handiest of weapons for Islamic fundamentalists, anti-Western, anti-American extremists and radicals of any stripe.

That's why, in the opinion of many knowledgeable authorities, the Arab League delegation is coming here. "All the moderates are threatened," says one, "and the Reagan initiative is the only show in town."

The key to Reagan's success will be in how well he recognizes the difference between what the serious Arabs want and what they can safely say they want, either collectively or individually, out loud. Much will depend, as well, on how precisely and persuasively the president is able to define his Palestinian position: as something strikingly new and different that would guarantee Israeli withdrawal from the West Bank and Gaza, or as a logical extension of Camp David, offering no firm promise of the federation with Jordan that is now an American "preference."

To paint it as no more than a "preference" risks dashing Arab expectations. The "moderates" want movement in exactly that direction, fast. But to picture it as a promise would simply solidify Israel's already rough rejection.

That is the bind the Reagan administration got itself into when it abandoned its honest broker's role in favor of a stated preference. Administration officials insist this was the necessary price for even a modicum of early interest in the Reagan initiative on the part of Hussein, and that the new U.S. position is less hard and fast than the positions of the other Camp David partners.

In that sense, it becomes one more argument for laying aside all prior claims and "preferences." That, after all, is the point of the Camp David formula: to temporize, to let time heal, to experiment with five years of "full autonomy" before tackling the hardest part having to do with the final fate of the occupied territories and their inhabitants.

So bring on the Trollope ploy. Ignore some of the more extravagant proclamations of sovereignty made by Israeli Foreign Minister Yitzhak Shamir and concentrate on what he wrote earlier this year in Foreign Affairs: Israel has "a claim to sovereignty over Judea, Samaria and Gaza. . . . The claim will undoubtedly be presented at the end of the five-year interim period. . . . There would be a comparable claim on the Arab side. . . . By that time one would hope that the kind of atmosphere will have been created that will make it possible to reach an agreement involving a solution acceptable to both sides."

In the same issue, King Hussein's brother, Crown Prince Hassan, wrote that an Arab-Israeli settlement that did not resolve "Israel's or Jordan's or Lebanon's or Syria's right to exist with reasonable security within a recognized territory . . . would be no settlement at all." That might not be a flat-out recognition of Israel. But surely it's a squeeze of the hand that ought to be required of any Arab who professes an interest in peace.

It is a lot to ask of the Arab moderates that they play the card of formal recognition of Israel while Israel holds the card of physical occupancy of the land in dispute. But it is not too much to ask for less insistence by all parties, including the United States, on the nature of the second Camp David step before the first step (autonomy) has been taken. graphics /illustration: Fair-weather friends By BAS