River taking sulphur to Sumerian city-state (5)
I believe the answer is:
stour
'river' is the definition.
(East Anglian river)
'sulphur to sumerian city-state' is the wordplay.
'sulphur' becomes 'S' (synonyms).
'sumerian city state' becomes 'Ur' (ancient city).
's'+'to'+'ur'='STOUR'
'taking' is the link.
(Other definitions for stour that I've seen before include "River Constable painted" , "Harwich's river" , "English river" , "Constable country river" , "Essex/Sussex border river" .)