Sitting Bull's tribe (5)
I believe the answer is:
sioux
'sitting bull's tribe' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sioux that I've seen before include "Amerindian" , "N. American natives" , "Native American people" , "Native American from between Lake Michigan and the Rockies" , "Native American of Minnesota, perhaps" .)