Tributary of the Missouri (5)
I believe the answer is:
osage
'tributary of the missouri' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for osage that I've seen before include "Brave" , "Indians" , "Amerindian" , "People of America" , "Age so (anag) - member of a Native American people" .)