A clan was a group of people bound together by (factual or perceived) common kinship and descent.
Zobral's species was divided into clans, additionally separated by a caste system, during the 22nd century. By 2152 the castes had been abolished, but the Torothan clan still controlled the government and "everything else", while Zobral's clan fought against their dominance. (ENT: "Desert Crossing")
Until the 23rd century, the Acamarian were divided into clans which often fought each other in long, vengeful feuds. One such feud was between clans Lornak and Tralesta. For the Gatherers, clan membership still played an important role in the 2360s. (TNG: "The Vengeance Factor")