Son needs to mature to become a wise man (4)
I believe the answer is:
sage
'a wise man' is the definition.
(sage is a kind of wise man)
'son needs to mature' is the wordplay.
'son' becomes 's'.
'needs' says to put letters next to each other.
'to mature' becomes 'age' ('age' can be a synonym of 'mature'**).
's'+'age'='SAGE'
'to become' acts as a link.
(Other definitions for sage that I've seen before include "A herb or a guru" , "Herb used in cooking" , "Culinary herb" , "Wise man - flavoursome plant" , "Wise woman" .)