Keen son with stringed instrument (5)
I believe the answer is:
sharp
'keen' is the definition.
(I know that keen can be written as sharp)
'son with stringed instrument' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'with' says to put letters next to each other.
'stringed instrument' becomes 'harp' (harp is a kind of stringed instrument).
's'+'harp'='SHARP'
(Other definitions for sharp that I've seen before include "Hurtful" , "needing strings slackened?" , "Acidic; pointed" , "Tangy" , "Clearly defined; acidic" .)