Intellectual adult leaving disarmament in confusion (10)
I believe the answer is:
mastermind
'intellectual' is the definition.
(I know that mastermind is a type of intellectual)
'adult leaving disarmament in confusion' is the wordplay.
'adult' becomes 'a' (abbreviation).
'leaving' indicates named letters should be taken away.
'in confusion' indicates anagramming the letters.
'disarmament' with 'a' taken out is 'disrmament'.
'disrmament' with letters rearranged gives 'MASTERMIND'.
(Other definitions for mastermind that I've seen before include "Brainy TV quiz" , "TV knowledge contest" , "Knowledge-testing TV quiz" , "Criminal genius" , "authority" .)