Missouri River tributary (5)
I believe the answer is:
osage
'missouri river tributary' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for osage that I've seen before include "Amerindian" , "Missouri tribe; go sea (anag.)" , "member of tribe" , "Age so (anag) - member of a Native American people" , "American Indian" .)