Mature woman's first in set (4,2)
I believe the answer is:
grow up
'mature' is the definition.
(growing up is a kind of maturing)
'woman's first in set' is the wordplay.
'woman's first' becomes 'w' (1st letter of 'woman').
'in' indicates putting letters inside.
'set' becomes 'group' (group is a kind of set).
'w' inserted into 'group' is 'GROW UP'.
(Other definitions for grow up that I've seen before include "Become mature, or an injunction to have sense" , "Informal injunction to have sense, be mature" , "Become an adult" , "Be your age, acquire maturity" , "Become mature - or injunction to act like it?" .)