The showdown over the future of proprietary trading by U.S. banks is about to begin. One likely fight: defining exactly what proprietary trading is. Wall Street has been hungry for details ever since President Barack Obama proposed curbs last month that would limit the ways banks with insured deposits bet with their own capital. Some answers will emerge Tuesday at a Senate Banking Committee hearing where former Federal Reserve Chairman Paul Volcker is set to testify. “There is a broad distinction between proprietary trading and responding to a customer requirement,” Mr. Volcker said in an interview…