clan (n.)

"a family, a tribe," especially, among the Highlanders of Scotland, a form of social organization consisting of a tribe holding land in common under leadership of a chieftain, early 15c., from Gaelic clann "family, stock, offspring," akin to Old Irish cland "offspring, tribe," both from Latin planta "offshoot" (see plant (n.)).

The Goidelic branch of Celtic (including Gaelic) had no initial p-, so it substituted k- or c- for Latin p-. The same Latin word in (non-Goidelic) Middle Welsh became plant "children."