Finished with organisation, thoroughly wicked (8)
I believe the answer is:
fiendish
'thoroughly wicked' is the definition.
'fiendish' can be an answer for 'wicked' (similar in meaning). I am not sure about the 'thoroughly' bit.
'finished with organisation' is the wordplay.
'with organisation' indicates anagramming the letters.
'finished' anagrammed gives 'FIENDISH'.
Can you help me to learn more?
(Other definitions for fiendish that I've seen before include "how diabolically cruel" , "Extremely cruel" , "Like Satan or the devil" , "Very difficult" , "Diabolic" .)