Pollute river that will carry scum miles away (6)
I believe the answer is:
defile
'pollute' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for defile that I've seen before include "passage" , "Create pollution" , "Pollute in ravine?" , "Sully; narrow gorge" , "Desecrate" .)