Key English river dredged at last, decontaminated (8)
I believe the answer is:
deloused
'decontaminated' is the definition.
The definition and answer can be both related to change as well as being past participle verbs.
Maybe you can see a link between them that I can't see?
'key english river dredged at last' is the wordplay.
'key' becomes 'del' (del key on a keyboard).
'english river' becomes 'ouse' (I've seen this in another clue).
'at last' indicates one should take the final letters.
The last letter of 'dredged' is 'd'.
'del'+'ouse'+'d'='DELOUSED'
Can you help me to learn more?
(Other definitions for deloused that I've seen before include "Made free of lice" , "free of pests" , "got rid of parasites" .)