In 1883, Clarke was sent to London, under the alias of "Henry Wilson", to take part in the Fenian dynamite campaign advocated by Jeremiah O'Donovan Rossa, one of the IRB leaders exiled in the United States. British authorities were already following those involved, aided by informants, and Clarke was arrested in possession of dynamite, along with three others. He was tried and sentenced to penal servitude for life on 28 May 1883 at London's Old Bailey. He subsequently served 15 years in Pentonville and other British prisons. In 1896, he was one of only five remaining Fenian prisoners in British jails and a series of public meetings in Ireland called for their release. At one meeting, John Redmond MP, leader of the Parnellite Irish National League, said of him: "Wilson is a man of whom no words of praise could be too high. I have learned in my many visits to Portland for five years to love, honour and respect Henry Wilson. I have seen day after day how his brave spirit was keeping him alive ... I have seen year after year the fading away of his physical strength".