Conclude in sudden demise (3)
I believe the answer is:
end
'conclude' is the definition.
(I've seen this before)
'in sudden demise' is the wordplay.
'in' indicates a hidden word.
'END' is hidden in the letters of 'sudden demise'.
(Other definitions for end that I've seen before include "Terminate in den" , "ambition" , "'Finish, cease (3)'" , "American footballer" , "Exterminate - object" .)