Clue: Pollute river that will carry scum miles away (6)
Answer: DEFILE
Explanation: Definition: Pollute
 DEE(a river in the British Isles – more than one with the same name) **containing**(that will carry) "film"(a thin layer over a surface, in this case of dirt or froth on a liquid/scum) **minus**(… away) "m"(abbrev. for "miles").