This is what George F. Black had to say about the Sinclair surname. Thought his assessment of their "clanship" interesting in light of recent discussions about origins.
"SINCLAIR. This Caithness surname is of territorial origin from St. Clare in the arrondisement of Pont d'Eveque, Normandy. The first Sinclairs in Scotland appear to have been vassals of the great territorial magnates, de Morville. Their first possession in Scotland was the barony of Roslin, near Edinburgh, which they held in the reign of David I (1124-1153). The earliest bearers of the name appear in charters connected with the abbeys of Dryburgh and Newbattle, the Hospital of Soltre (now Soutra in Midlothian), the church of Glasgow, etc. An early offshoot of the family became all powerful in Caithness and held the earldom there from 1379 to 1542. The frequency of the surname in Caithness and in the Orkneys is due to the tenants on the lands of the earldom adopting the name of their overlord just as we find tenants who possessed no surnames of their own doing likewise elsewhere.
"The Sinclairs, like the Gordons and some other families, cannot be called a clan in the true sense of the term. They were a powerful territorial family, whose relationship to their dependents was entirely feudal... An old rhyme referring to the bickerings between the Sinclairs and their neighbors says: 'Sinclair, Sutherland, Keith and Clan Gunn, There never was peace when thae four were in.'
"...In Argyllshire the name is used as an Englishing of Gaelic Mac na cearda."