They're obviously young son's first things? (4)
I believe the answer is:
toys
'things?' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps you can see a link between them that I can't see?
'they're obviously young son's first' is the wordplay.
'first' suggests taking the first letters.
The initial letters of 'theyre obviously young sons' is 'TOYS'.
(Other definitions for toys that I've seen before include "Things that provide amusement" , "Plays (with something), not seriously" , "Fiddles" , "Film" , "Trifles with playthings" .)