THIS FALL, American field trials and bird shoots are beginning to have a bit of the flavor of the African veldt and India's plains, but salted with the spirit of the Far West. It all began when a group of big-game-hunting friends sent Norm Thompson of Portland, Ore. a bush coat made by the famous safari outfitter, Ahmed of Nairobi. Thompson, a former advertising man who moved west and began raising Labrador retrievers and supplying outdoorsmen with all sorts of gear by mail, had been looking for something new in upland game clothing. He combined the flair of safari wear with practical ideas long tested in the West and brought out his Shikari clothes, named for India's tiger-hunting guides, the shikaris. For his white-hunter hat, Thompson used the fine beaver-fur felt of a cowboy's ten-gallon, made it into an Africa-tested style: wide brim (3¼ inches) to keep out sun and rain, an inside band designed to keep the hat on in the wind, a Stewart Granger dash of genuine leopard-skin band ($17.50).