Fiendish cryptic, but completed (8)
I believe the answer is:
finished
'completed' is the definition.
(I've seen this before)
'fiendish cryptic' is the wordplay.
'cryptic' indicates an anagram.
'fiendish' with letters rearranged gives 'FINISHED'.
'but' acts as a link.
(Other definitions for finished that I've seen before include "wrapped up" , "dressed" , "Done, over, ended" , "Ended. completed" , "exhausted" .)