An article in The Sydney Morning Herald has the creator of Cleverman, Ryan Griffen, discussing the traditional Aboriginal role of the Cleverman. The article paints a picture of the Cleverman as a shaman of sorts, an elder and master of magic within a local community. Griffen states
"Our cleverman is different from the actual cleverman that existed in many Aboriginal communities," he says. "The Dreamtime is about the past, present and future, and so he has powers that relate to all three of those.
. . .
"In our show, these powers are passed down, very much as in Aboriginal culture. A cleverman is chosen by another cleverman when they're of the right bloodline or are the right type of person. For our cleverman, those powers are passed down to him by his uncle."
I agree that finding references to the Cleverman not connected to the show is extremely difficult. I found a tenuous reference in James Cowan's book Myths of the dreaming: interpreting Aboriginal legends (from an article here) connecting them with the mekigars, shamans.