Pollute Cheshire river with a lot of stuff in it (6)
I believe the answer is:
defile
'pollute cheshire' is the definition.
'defile' can be an answer for 'pollute' (I've seen this in another clue). I'm unsure of the 'cheshire' bit.
'river with a lot of stuff in it' is the wordplay.
'river with' becomes 'dele' (I can't justify this - if you can you should believe this answer much more).
'a lot of' means to remove the last letter (most of the word but not all of it).
'stuff' becomes 'fir' (fir is a kind of stuff).
'in it' means one lot of letters goes inside another.
'fir' with its last letter taken away is 'fi'.
'dele' going around 'fi' is 'DEFILE'.
Can you help me to learn more?
(Other definitions for defile that I've seen before include "Fed lie to pollute" , "Contaminate" , "valley" , "Create pollution" , "Sully; narrow gorge" .)