Adulthood (8)
I believe the answer is:
maturity
'adulthood' is the definition.
(I know that adulthood can be written as maturity)
This is the entire clue.
(Other definitions for maturity that I've seen before include "State of full growth or adulthood" , "State of having reached full development" , "Ripeness" , "age" , "Fool" .)