Tributary of the Missouri River (5)
I believe the answer is:
osage
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tributary of the missouri river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)