Fool of a son taken in by knaves (7)
I believe the answer is:
jackass
'fool' is the definition.
(jackass is a kind of fool)
'a son taken in by knaves' is the wordplay.
'son' becomes 's'.
'taken in by' indicates putting letters inside.
'knaves' becomes 'jacks' ('jack' can be a synonym of 'knave').
'a'+'s'='as'
'as' placed within 'jacks' is 'JACKASS'.
'of' is the link.
(Other definitions for jackass that I've seen before include "One may lark about" , "Asinine person" , "Male donkey; idiot" , "Charlie" , "See 7" .)