From the game's encyclopedia
Clans are a Government Form in which the amount of Levies and Taxes given to a Liege by their Vassals is determined by the Aptitude of said Liege's Tax Collector. Clans also make use of House Unity.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Clan is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Clans are a Government Form in which the amount of Levies and Taxes given to a Liege by their Vassals is determined by the Aptitude of said Liege's Tax Collector." It is part of Government. It covers Tax Jurisdiction. The game also links it as Clan Government and Clans. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.