Tributary of the Missouri River (5)
I believe the answer is:
osage
'tributary of the missouri river' 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 "Missouri tribe; go sea (anag.)" , "member of tribe" , "Indian people" , "Member of the Native American people, once occupying the Missouri valley" , "Amerindian" .)