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