McClave as far as I have studied it, is a sept of the Stewart of Appin clan that was located from around the Firth of Clyde in SW Scotland (a Sept, if you are not familiar with the term, is a family loyal to a clan or clan chief).This would correlate with the other post regarding Wigtown. The Stewart of Appin Clan was a poor relative of the Royal Stewarts.
According to a family tree assembled by one of my relatives over forty years ago, our family is descendent from Thomas McClave who departed Wigtown in the mid 1740's around the time of the Highland Clearings.The appropriate tartan for McClaves is Stewart of Appin of which there are several (dress, hunting, ancient, etc.).
My grandfather is Philip Lonie McClave, originally from Fort Lupton, Colorado (now a suburb of Denver).It was either his father or grandfather who founded the small farming community of McClave in the SE corner of Colorado.It is on the edge of the Great Plains.
My grandfather moved to Los Angeles in 1937 after his graduation from the school of veterinary medicine at Colorado State.My father was born there, as was I.In recent years I have done some internet research on McClaves.There are concentrations of them in New York, Pennsylvania, Georgia, Washington, Colorado, Oregon, and Northern Ireland.My relatives are mostly in Colorado and California.
As an amusing side note, I had not met any McClaves outside of my own family until about ten years ago when I had moved from San Diego to Las Vegas.It was there that by happenstance I met another Scott R. (Ross) McClave whose family originated from Pennsylvania.Pretty amusing, isn't it? I have since moved back to California.
I hope all the other info gives you something to work with.