Adult with note goes into class (5-2)
I believe the answer is:
grown up
'adult' is the definition.
(grown-up is a kind of adult)
'with note goes into class' is the wordplay.
'with' becomes 'w' (abbreviation).
'note' becomes 'n' (abbreviation).
'goes into' indicates putting letters inside.
'class' becomes 'group' (grouping is a kind of classing).
'w'+'n'='wn'
'wn' going into 'group' is 'GROWN-UP'.
(Other definitions for grown up that I've seen before include "Mature" , "Fully-developed" , "a fully developed one?" , "Adult" .)