Mature women in class (4,2)
I believe the answer is:
grow up
'mature' is the definition.
(growing up is a kind of maturing)
'women in class' is the wordplay.
'women' becomes 'w'.
'in' means one lot of letters goes inside another.
'class' becomes 'group' (grouping is a kind of classing**).
'w' inserted within 'group' is 'GROW UP'.
(Other definitions for grow up that I've seen before include "Become mature - or injunction to act like it?" , "Be your age, acquire maturity" , "Become an adult" , "Develop, become mature" , "Become mature, or an injunction to have sense" .)