Son with desire is impressively smart (5)
I believe the answer is:
swish
'impressively smart' is the definition.
(I've seen this before)
'son with desire' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'with' is a charade indicator (letters next to each other).
'desire' becomes 'wish' (wishing is a kind of desiring).
's'+'wish'='SWISH'
'is' acts as a link.
(Other definitions for swish that I've seen before include "Extravagant" , "Stylish (sl.)" , "Impressively smart" , "Impressively fashionable" , "Smart, fashionable" .)