Grown-up boy (3)
I believe the answer is:
man
'grown-up boy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for man that I've seen before include "Adult human male" , "Isle of -; chess piece" , "Geezer" , "The most destructive and polluting animal" , "Dude" .)