A Czechoslovak underground publisher was jailed for 2 1/2 years, the longest prison term handed down for dissident activities in Czechoslovakia in recent years, a leading activist said. The sentence for Frantisek Starek was announced as police warned Vaclav Havel, the country's best known dissident, that he runs the risk of being sent back to prison. Havel was freed from prison in May after serving half of an eight-month jail term. Starek was convicted of incitement, according to Petr Uhl of the Charter 77 human rights movement. Starek, the editor of the underground journal Vokno (Window) was arrested in February.