Male child (3)
'male child' is the definition.
(I know that male child can be written as son)
This is the entire clue.
(Other definitions for son that I've seen before include "Male child" , "The kind of child" , "Young boy" , "Male scion" , "Male offspring" .)
(Other definitions for boy that I've seen before include "Son" , "Youth" , "Lad" , "Young male" , "Youthful male" .)