Key English river dredged at last, decontaminated (8)
I believe the answer is:
deloused
'decontaminated' is the definition.
The answer and definition can be both related to change as well as being past participle verbs.
Perhaps they are linked in a way I don't understand?
'key english river dredged at last' is the wordplay.
'key' becomes 'del' (Delete on a keyboard).
'english river' becomes 'ouse' (I've seen this before).
'at last' suggests the final letters.
The final 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" .)