I am an active member of Clan Donnachaidh worldwide; I don't believe that there is an actual Clan Robertson. Robertson is one of the three principal septs of Clan Donnachaidh. The other two septs are Duncan and Reid (Reed). Altogether there are at least 50 septs affiliated with Donnachaidh.
More information can be found at www.robertson.org
which is maintained by Mike Duncan, a clansman in the mid-Atlantic branch of Clan Donnachaidh.