When son can be a fool (3)
I believe the answer is:
ass
'a fool' is the definition.
(ass is a kind of fool)
'when son' is the wordplay.
'when' becomes 'as' (**).
'son' becomes 's'.
'as'+'s'='ASS'
'can be' acts as a link.
(Other definitions for ass that I've seen before include "Donkey or foolish person" , "Horse hybrid" , "animal inside" , "Animal; fool" , "Pompous fool or beast of burden" .)