Grown-up isn't right (5)
I believe the answer is:
wrong
'isn't right' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'grown-up' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'grown' with letters rearranged gives 'WRONG'.
Can you help me to learn more?
(Other definitions for wrong that I've seen before include "Incorrect, illegal or immoral" , "Not correct" , "Injury - incorrect" , "Treat unjustly" , "Incorrect - sinful" .)