Small boy is unable to sing (7)
I believe the answer is:
descant
'sing' is the definition.
(descant could historically mean to sing)
'small boy is unable' is the wordplay.
'small boy' becomes 'des' (short for Desmond; 'small' asks for a short form).
'is unable' becomes 'can't' (if you can't do something, you're unable to do it).
'des'+'cant'='DESCANT'
'to' acts as a link.
(Other definitions for descant that I've seen before include "Sung variation" , "Independent melody sung above a basic melody" , "Independent treble line" , "Melody sung or played above a basic melody" , "Ornamental melody added above a theme" .)