Clearchus was a citizen of Heraclea on the Euxine who was recalled from exile by the oligarchy of that city to aid them in quelling the growing discontent and demands of the people. According to Justin, Clearchus reached an agreement with Mithridates of Cius to betray the city to him on the condition that Clearchus would hold the city for Mithridates as governor. But, Clearchus then came to the conclusion that he could make himself master of the city without the aid of Mithridates. So he not only broke his agreement with the Mithridates, but also captured him and compelled him to pay a large sum for his release.