I'm returning south, bitter with one Midwestern US state (8)
I believe the answer is:
missouri
'midwestern us state' is the definition.
(I've seen this before)
'i'm returning south bitter with one' is the wordplay.
'returning' says the letters should be written in reverse.
'south' becomes 's' (abbreviation).
'bitter' becomes 'sour' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'im' in reverse letter order is 'mi'.
'mi'+'s'+'sour'+'i'='MISSOURI'
(Other definitions for missouri that I've seen before include "Biggest tributary of the Mississippi" , "Flower" , "River joining the Mississippi south of St Louis" , "US river - state" , "Midwestern US state" .)