Somewhat fiendish purpose (3)
I believe the answer is:
end
'purpose' is the definition.
(both can mean a goal)
'somewhat fiendish' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'END' is hidden in the letters of 'fiendish'.
(Other definitions for end that I've seen before include "Terminus" , "Finished" , "Complete" , "Final part" , "Conclusion or goal" .)