A good deal of harking back to the supposedly good old days of the 1920s is evident in new men's clothes for fall. It is only fair to add, though, that the nostalgia has been tempered by a thoroughly modern approach in cut, subtlety of color, and depth of texture.

Double-breasted suits, silk shirts, Norfolk jackets, turned-back trousers, fedora hats, and French cuffs have all been enthusiastically revived. But the reincarnations do not necessarily bring images of Jay Gatsby (nor for that matter, Sebastian Flyte and his teddy bear) instantly to mind. The essential appeal of the period clothes so greatly admired in ''Brideshead Revisited'' is that of elegance. So the call in men's fashions now is for heightened standards - even the patterned sweater vest is to be worn with a well-chosen shirt and tie.

Although lines are longer, the attenuated look of this season's women's fashions is not so pronounced for men. Lapels and jacket lengths have gained only an inch or two. Elongation is most noticeable in sportswear (Calvin Klein's latest bomber jacket is, for instance, hip length) and in the return of three-quarter-length car coats.

Shoulders in the 1920s tended to be narrow. For 1982-83, shoulders are as wide as last season, but they have been softened in outline. Lapel widths are generally as they were. It is worth noting that double-breasted suits, which may have as many as six buttons - only two of which are actually meant to be buttoned - often sport peaked lapels, a remembrance of things past. The pleat-top trouser with a straight narrowed leg remains in favor. Cuffed trouser legs are the current innovation.

For business, men will find many more smooth-surfaced suitings. Sharkskins and cheviots are back. Colorations in suitings, we are told by the Men's Fashion Association (a group to which every leading menswear manufacturer is allied, and a fount of knowledge on incoming trends), include many more medium tones as well as the usual darks. Stripes are the outstanding pattern, and the choice runs from hairline to chalk. Subdued Glenurquhart plaids and herringbones are also top style. The news is in the mixtures. Olive has been added to a palette that emphasizes blends of brown and blue, and brown with gray.

In keeping with the dressier mood of the hour, three-piece suits - some of them city-and-country types in tweed - are popular. Sports jacket fabrics are nappier this season and have more loft. Much is being made of the color mixes of gem tones - garnet, amethyst, lapis, and even turquoise - which result in some rich effects. For those with less adventurous tastes, there are plenty of camels , grays, and heather tones. Plain-color blazers exist in dozens of hues.

Some nostalgic touches used in new casual jackets are bi-swing backs, gusset shoulders, bellows pockets, and gun patches.

Topcoat styles follow the gentlemanly trend, too. The chesterfield, the camel-hair polo coat, and the raglan-shouldered balmacaan are in fine fettle these days.

In casual outerwear, the bomber jacket remains the No. 1 chic shape, but down-filled garments are in the majority. Some 75 percent of all men's winter warmers are either down- or fiber-filled quilting. Shearling is another leader in this category. The bigger and longer these coats are, the higher the fashion quotient - broad shoulders and roomy armholes contribute to the recommended oversize look.

Sweaters are a further alternative for beating the chill factor. Machine-mades with a hand-knit appearance rate high in a vast assortment that includes shawl-collared and shaker styles along with the customary pullovers and cardigans. The argyle motif, enlarged to cover the whole chest, or used as a stripe down one side of the sweater, has received a lot of attention.

As to shirts, the latest is a colored or patterned style with a white collar that may be button-down or spread, rounded or pointed. Colored shirts are available in deeper tones than heretofore, and those who fancy a burgundy or moss green will have no trouble finding it.

Shirts with French cuffs may presage a return to cuff links for day wear - who knows? Wearing a silk shirt in the daytime might strike some men as suggesting too much a riverboat-gambler. But the same men would have to agree that a pure white silk would be elegance itself with a dinner jacket.

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