ANYONE WHO joins a president's Cabinet in the seventh year of an eight-year administration is likely to find that the moment for exciting new departures has passed. That is doubtless going to be the case for C. William Verity, whom President Reagan has nominated to be secretary of commerce. He is going to spend most of his time on the agenda that he inherits. But that agenda is not unimportant, nor are the crucial issues on it the most obvious ones.

The trade bill now in conference will absorb a lot of his attention, but he has arrived too late in the process to play much of a part in it. Similarly, the tensions with Japan over trading practices are necessarily a great preoccupation of any secretary of commerce, but at this point others in the administration are more deeply engaged. Mr. Verity's chief contributions will probably lie elsewhere.

He will be a central figure in the continuous guerrilla warfare with the Pentagon over export controls. The Pentagon keeps doggedly trying to expand the controls to more and more goods, frequently including those that are easily available abroad. These controls have become a major hindrance to foreign sales of the kind of advanced technology at which this country excels. Commerce has argued for years that export controls are most effective when they are limited to a short list of truly crucial items that the government can track with precision. Mr. Verity's views here are very much in the Commerce tradition, and since Commerce is absolutely right on this one, he can do a great service by pressing energetically for a more rational export-control policy.

Mr. Verity comes from the steel industry -- he's a former chairman of Armco -- and his experience there may prove useful. A lot of steel mills have closed, and more are going to have to close. Some of the implications for the federal government and its budget are illustrated in the current struggle between the bankrupt LTV Corp. and the federal Pension Benefit Guaranty Corp. over the pensions of retired LTV steelworkers. The PBGC is now insolvent by the impressive margin of $4 billion, most of it because of the pension liabilities that the government has inherited from LTV. The Reagan administration is profoundly reluctant to get more deeply involved in the troubles of the big steel companies, but it may not be able to afford to avoid a more active part. Massive change is overtaking American heavy industry, and even in an administration that believes in leaving change to the market, the secretary of commerce is a key figure in the government's response.