Enviros are upset over what they fear may be an impending cave-in by the Clinton-Gore administration on the issue of mountaintop-removal mining in West Virginia. A federal judge ruled last year that the destructive practice, in which mining companies blast off the tops of mountains to get at coal and then bury nearby streams with the debris, violates the Clean Water Act and a federal mining law. But West Virginia is appealing the ruling, and it seems the Clinton administration may be preparing to file a brief in support of the state, egged on by powerful West Virginia Sen. Robert Byrd (D). A coalition of 21 enviro groups yesterday sent President Clinton a letter imploring him to stand firm against mountaintop-removal mining, as did 23 moderate House Republicans last month.