It may take a few more days for Kansas City fans to take seriously smooth, decisive 7-2 victory that their Royals laid on the New York Yankees here today as a introductory chapter to the drama of the American League playoffs. After all, elation is so fleeting when measured against the oppressive reign that the Yankees have excercised over this town for a quarter of a century. Kansas City won't believe their Royals have exorcised the Yankee hex until they see the first World Series pitch. Or maybe the second.

However, the Yankees understood the true meaning of the Royal show of strength on this summer afternoon misplaced in autumn. What they saw was not just a shaky 10-hitter by Royal southpaw Larry Gura. Nor was it the two-out, two-run hits by Frank White and Willie Mays Aikens that drove Ron Guidry from the game after only three innings. Even the Kansas City insurance runs, on a solo homer by George Brett and a 410-foot two-run double by Willie Wilson, were not impressive in themselves.

What the Yankees perceived was the whole upsettling shape of this afternoon -- the impression of a Royal team that thinks it has the New Yorker's number. Kansas City has finally found a concoction to poison the New York monster sick. It remains for the Yankees to find an antidote.

"They beat us exactly the way they beat us all year," said Yankee Graig Nettles. "It looked very familiar."

"They hammered us, like they've hammered our pitching all season," said Reggie Jackson, who failed to hit in four at-bats and stranded five runners. "They gave us a good beating. They're clearly the best Kansas City team we've played."

What combination of ingredients makes this particular Royal team so toxic to these Yankees? In 13 games this year, nine of them K.C. victories, the Royals have scored 96 runs and outscored New York by 38 runs.

The Yankee-killing potion -- a finely balanced and by no means foolproof mix -- was the same this afternoon. Let a Royal veteran leader explain.

"We have to play around their bullpen, especially Goose Gossage. And we have to stay out of the kind of one-run, back-and-forth game that becomes a depth contest," said Hal McRae. "That means we have to score early against their starters and take a lead. We have to put the ball in play -- avoid routine fly balls -- because if we force their outfield to make a lot of good plays, they probably won't. We can steal on them and take the extra base.

"The biggest thing is not to go into the late innings behind. That's their game," said McRae, aware that the Yankees have a 77-2 record in games in which they led going into the seventh inning. "If you have to face Gossage, then force them to bring him in early so he'll tire a little, or at least work enough innings so that they can't use him for a day or two."

McRae's description fits this contest. The Yanks back-to-back homers in the second inning by Rick Cerone and Lou Piniella, into the bleachers in the left field corner for a 2-0 lead, off slow Gura curves. But that was the last thing that went according to the Yankee plan.

In the second, after a single, a walk and a wild pitch. White hit a two-out fly to left. On the previous play, slow Piniella had made a fine sliding catch of a similar keep-the-ball-in-play bloop. This time, Piniella got a bad jump, lumbered in and didn't arive in time. The two-run pop double landed in front of him to tie the game. "That ball has to be caught," said Guidry. "My job is to get a man to hit a pop-up. I did my job."

In the next inning, the impressive Royal regulars, who have not been this healthy all year, were banging on Guidry's door again, loading the bases with two out. Aikens, a .199 hitter against southpaws this season, who started only on a hunch by Manager Jim Frey, laced a 3-1 slider into left for a two-run hit that gave K.C. the lead it finds so vital.

At that juncture, the Royals had their best piece of luck. Gura, who had not won a game since Aug. 24, began the day like a man looking for disaster. Though his career record against New York is great (71), his playoff mark is abominable. Of the first dozen Yankees batters this day, three had doubles and two homers. At that moment, the Yankee batting average off Gura in four playoffs was .380.

Then the shadows arrived, cutting across the field between home and the rubber. For a speed changer, in particular, it was a heaven-sent light; identifying pitches quickly is impossible in bad shadows. Thereafter, Gura, who had escaped with Yankees on third to end each of the first three innings, allowed just one other man past first base, in the seventh, when Jackson stepped up with men at the corners.

For Gura, this was a moment of personal triumph and vindication.In the ABC-TV booth was color commentator Billy Martin, who, when he managed the Yanks in the playoffs, said of Gura, "The only way we can lose to the Royals tomorrow is if Gura gets hit by a car and can't get to the park. We should send a limousine for him."

In Martin's book, Jackson against Gura with a playoff game on the line was Mr. October against Mrs. October, but Frey thought differently. Instead of asking Gura if he wanted to face Jackson, Frey said, "Unless you're tired, I want you to pitch to Jackson. Just don't let him hit it in the damn bullpen."

For the fourth time, Gura made Jackson look feeble, this time on a grounder to second.

"I didn't think of Martin all day until after the inning I got Jackson out," said Gura. "In the eighth, I said to myself, 'I wonder what Billy's thinking up there."

Jackson was thinking he wished it were night, not evening. "Don't like shadows . . . Don't like the kinda mean pitches Gura was throwing, either. Hell, we're a night ballclub, right?," he said to surrounding Yankees. "The old-timers on this team need to sleep late. We don't get to movin' around good until sundown."

The light didn't bother Brett, whose homer in the seventh -- a 400-foot opposite-field launch through a crosswind on a perfect low and away fast ball -- efectively iced the game. Brett now has 23 Rbi in 10 games against New York in '80, a pace that would produce 373 RBI in a 162-game season.

"They're pitching around me," said Jackson. "I wish we'd pitch around him (Brett) a little more. But then, I don't know anything about pitching, or defense. I just know about slugging. And today, it didn't look like I knew much about that."