Former San Antonio Mayor Henry G. Cisneros is the latest politician to discover that being a second-string Cabinet member isn't much fun. Like his predecessor at the Housing & Urban Development Dept., Jack F. Kemp, Cisneros is an energetic booster of an aggressive new urban policy. Also like Kemp, he's having a hard time getting his President's attention. Cisneros' ambitious enterprise-zone plan was reduced to a shadow of itself in the proposal issued by the White House on May 4. He got nowhere with pleas that Bill Clinton fight harder to save community-development block grants from the Republican assault on the Administration's stimulus plans. In recent weeks, Cisneros has been asking mayors, municipal-bond underwriters, and other urban interest groups to lobby the White House on his behalf.