Son's desire to be smart (5)
I believe the answer is:
swish
'be smart' is the definition.
(thesaurus)
'son's desire' is the wordplay.
'desire' becomes 'wish' (wishing is a kind of desiring).
's'+'wish'='SWISH'
'to' acts as a link.
(Other definitions for swish that I've seen before include "Impressively fashionable" , "Smart; stylish" , "Elegant" , "Stylish (sl.)" , "Smart, fashionable" .)