The ultimate in fiendishness (3)

I believe the answer is:

end

'the ultimate' is the definition.
(thesaurus)

'in fiendishness' is the wordplay.
'in' says the answer is hidden in the clue.
'END' can be found hidden inside 'fiendishness'.

(Other definitions for end that I've seen before include "American footballer" , "Time to break-up (3,2,4) - See 12" , "Finish, wind up" , "Concluding purpose" , "Unyielding" .)

I've seen this clue in the Evening Standard.

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