Saying the move would protect tens of thousands of acres of wetlands a year, the Clinton administration yesterday proposed closing a loophole that allows wetlands to be developed. The Clean Water Act restricts developers from filling wetlands to create stable land on which to build and farm, but does not specifically prevent developers from draining wetlands to do the same. In 1993, the administration issued a regulation to forbid such draining, but the courts threw it out as too broad. The new rule, now open for public comment for 60 days, would go a long way toward shifting the burden of proof onto developers, forcing them to prove that their projects wouldn’t harm wetlands. But administration officials said it would take an act of Congress to close the loophole entirely. Environmentalists praised the proposal, while developers, predictably, were not happy clams.