WASHINGTON - A special prosecutor's long-delayed report charges that a coverup at senior levels of the Clinton administration killed a tax fraud case against ex-cabinet member Henry Cisneros, the Daily News has learned. David Barrett's 11-year, $23 million probe, which will be released tomorrow, states in stinging terms that this Clinton coverup succeeded. Cisneros was forced to admit in 1999 that he had made secret payments to a mistress before serving as Clinton's secretary of Housing and Urban Development. Barrett investigated tax fraud charges stemming from those under-the-table payments. Then-IRS Commissioner Peggy Richardson, a close friend of Sen. Hillary Clinton...