Looking ahead to easygoing elegance for fall fashions

Having had a fling last season with super-fitted clothes, America's designers are in effect admitting the error of their ways. For fall, they have done an about-face by reverting en masse to the kind of fashion they've always done the best: beautifully relaxed, unconfining sportswear.

New styles for autumn, presented here recently, are still long and lean, with accentuated shoulders. But the tight torso look, which was difficult to wear and met with considerable customer resistance, has been put on the shelf.

What came out on the runways at the advance showings were for the most part easygoing yet elegant versions of the genre of fashion that has become famous the world over as the American Look. Seventh Avenue was awash with cashmeres, gray flannels, suedes, polo coats, glen plaid and houndstooth check tweeds, mariners' pea jackets, lightweight ponchos, and every form of sweater dressing known to womankind.

The casual but nonetheless slicked-up approach applies for all times of day. Even such a proponent of rich after-dark glitter and glamour as Bill Blass is advocating the sportive alternative for evening: a hot-color satin trenchcoat over a chenille velvet pullover with pleat-top flannel trousers.

Both his Blassport separates and his couture-level collections emphasize more elongated proportions, a trend followed by most of his colleagues for fall. The below-hip-length cardigan with a narrow skirt or pants will, however, be topped by a very roomy broad-shouldered coat. As often as not, this will be reversible. The Blass black-and-white Prince of Wales tweed can, for example, be turned about to plain luggage-tan wool.

If black and white checks, cashmeres, gray flannels, et al. seem to have a familiar ring, it should be noted that the current treatments are not in the old tried-and-true classic mold. Fashion marches on, as it were.

These updated plaids are likely to be giant enlargements of anything seen heretofore. Calvin Klein's tartans, used for sweeping capelet-style capes with pipestem Scottish trews, are larger than life. He's done slouchy, oversize blazers of the tartans, too. His Glenurquhart patterns, made into enveloping ankle-long greatcoats and worn with coordinated skirts, appear to have square-foot dimensions.

Klein's gray flannels aren't the usual, either. They have black grosgrain stripes down the sides - which accounts for his calling them ''tuxedo pants,'' no doubt.

As for his sweater dresses, the niftiest numbers are belted coat styles with toggle fasteners - the duffel idea translated in hunter green, bordeaux, or vicuna knit. The news in nighttime sweaters at Klein is pure and simple: black V-neck cashmeres with bands of sequins at the neckline, wrists, or hipline, worn with black wool dinner pants. Trousers, by the way, have been reinstated as a top-priority fashion for fall.

Above all, this is the year of the sweater, and every designer has his own way with it. Ralph Lauren opened his show, held in the vast Seventh Regiment Armory, where the audience sat on bleachers with families of skiers. His men, women, and children trooped down the catwalk wearing bright wools embroidered with fir trees and downhill racers.

Some of Lauren's new Tyrolean jackets are suede - a material he also lavishes on loose tunics with big cowled turtlenecks - but other jackets are knit replicas of what the Austrians wear in their Alps, from authentic down to the staghorn buttons. Lauren makes a big point of cashmere for evening as well as white-collared coat dresses aimed at the Wall Street customer. New tailored flannel dresses include a banker's gray wrap style.

At Albert Nipon, the dress continues to reign supreme, with chemise and coat styles decked with pleated ruff or choirboy collars in the fore. Nipon does have an ample assortment of suits with companionable coats. Cropped jackets with puffed shoulders with Scots plaid skirts (there's a choice of straight or flared) have feminine dressmaker touches. Leather is used for skirts as well as for bindings on big steamer coats. Knits are well represented, too.

Sweaters here come in Aztec patterns that combine violet, rust, and dark green, or fuchsia, luggage, and mauve. Pairings of fuchsia or cyclamen with what are thought of as traditional autumn hues is a new scheme for the coming fall. It would not, therefore, be wise to take that fuchsia blouse, bought three years ago when intense purples were all the rage, to the cleaners to be dyed. Fuchsia has a fashion future.