Its capital is Jefferson City (8)

I believe the answer is:

missouri

(Other definitions for missouri that I've seen before include "River joining the Mississippi south of St Louis" , "Biggest tributary of the Mississippi" , "US state, capital Jefferson City" , "American state's" , "US river - state" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.