I am looking for the clan to which the family name MacSkimming belongs.

From "The surnames of Scotland" by G.F. Black p. 564:

"..first know reference to a McSkimming, Thomas Mackskimming in Auchreicht 1684. The name occurs in GALLOWAY in 1684 as McSeumin, McSimyn, and McSkimming (parish). David McSkimming in Cairandlarie 1757 (Wigtown)."