The latest US trade deficit - a whopping $7.23 billion for the third quarter of 1981 - should be taken as both a warning and a reminder. It should alert policymakers to be unflagging in their efforts to promote the export of American products abroad, as well as unhesitatingly resist moves toward protectionism by any nation. It should also serve as a reminder that the American economy cannot be separated from the larger world economy, and that the very rise in the value of the dollar that has so pleased some administration officials has also made US goods more expensive, thus working against US exports.
The US, as the world's largest single economic system, must take the lead in propelling vigorous world trade. Such trade is now more important than ever, what with the downturn in the economies of Europe and the United States and rising unemployment. The challenge for the Reagan administration will be not to let the necessity of expanding trade be lost in the press of domestic difficulties.
The current nine-month deficit of $18 billion is already running about equal to that registered in the same period last year. Any major falloff in the current fourth quarter could easily lead to a larger deficit than last year, and possibly even a record deficit. In fact some administration officials now believe that the US ''current account balance'' - a broader measure that includes overseas trade in services as well as goods - will post a deficit this year, rather than the more normal surplus. And, looking ahead, the administration is already conceding that next year's trade gap with Japan, in particular, could jump sharply, spurring demands for new restraints against imports.
The US Commerce Department, the Chamber of Commerce of the US, and other business groups must find new ways to encourage fledgling and smaller firms to enter the foreign trade market. Most US firms are not yet marketing goods beyond national borders. The administration should also reconsider its proposed cutback in the Export-Import Bank, which facilitates sales of ''big-ticket'' products, while at the same time following through in its negotiations with other nations to reduce government subsidies of export industries. It might also consider centralizing the entire foreign trade apparatus, now scattered in a number of agencies.
More controversial is the question of whether the federal government should at some point ''intervene'' in exchange markets to modulate the dollar's value as a way of aiding exports. Treasury Undersecretary Beryl Sprinkel has said that it would be very ''unlikely'' for the Reagan administration to intervene in international financial markets. But even Mr. Sprinkel seemed to be leaving the door ajar somewhat by commenting that intervention to adjust the dollar could come ''if highly volatile, extremely disorderly markets and wild swings in exchange rates'' developed. Such an approach seems reasonable.
In short, the United States must make every effort to promote a strong - and expanding - world trade.