THE cleavage is back make no mistake. And what is making it possible is the return of the bra. Cleavage, once the domain of barmaids, porn stars and D cups is now back in business. As the leg and ankle disappears under flares and long skirts, the bosom is being restored as a fashion accessory. Designers agree this is a modesty they are going to have to overcome. Of course, we should have seen it coming. Madonna, in her videos, was seen tilting her giant-sized ice cream cones into our faces. Supermodels strutted the catwalks, and breast-fetishism was revealed in the exposed corsetry of Italian designers Dolce & Gabbana. It has resulted in the spectacular revival of what is almost considered the grande old lady of lingerie, the Gossard Wonderbra. Born in the '60s, the Wonderbra is more '50s in image. Covered in opaque lace, it is underwired with a deep front plunge decorated with a flirtatious bow. Since last November sales for the greatest uplifter in the bra business have risen by 50 per cent. Gossard is allegedly frantically trying to keep up production to match demand. In terms of bra engineering the Wonderbra comprises 42 separate components involving 26 different sewing techniques. The bra has steel wires coated in plastic - heavier than the ones used in ''modern'' bras - for maximum uplift; strong, wide, elasticated ''wings'' to give a good cleavage effect; and criss-cross back straps to keep the bra high on the rib cage. The bra has become a vital accessory due to the supermodels. Most of them have undergone plastic surgery to match up to the new fashionable proportions. Those without the cash or the inclination to follow suit have the alternative of a padded bra. This is also a safer option following recent medical scares over silicon implants. The Wonderbra does, however, have its rivals. Marks & Spencer has a version which is almost as good. The lingerie giant reports that padded or not, underwired styles account for half of its bra sales. Like Gossard, other companies are seeing their old styles enjoy a revival. Playtex's Superlook Body Language with ''Cookies'' (the company's name for removable pads), which was launched in 1971, is a best seller once again. So after 10 years of being seen as a tacky and old-fashioned lacy confection, the bra has been given designer status. Gossard's Wonderbra is available in Hongkong by mail order from Jennifers, the Lingerie Shop (phone 868-2225) priced $219 plus $12 postage and packing. The Marks & Spencer version is available at the Pacific Place branch. Model: Sally from Models Four.