Intelligence of son, quietly in control (9)
I believe the answer is:
sharpness
'intelligence' is the definition.
(I know that sharpness is a type of intelligence)
'son quietly in control' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'quietly' becomes 'p' (p for piano and quiet).
'in' is an insertion indicator.
'control' becomes 'harness' (harnessing is a kind of controlling).
'p' placed within 'harness' is 'harpness'.
's'+'harpness'='SHARPNESS'
'of' acts as a link.
(Other definitions for sharpness that I've seen before include "Being quick to perceive" , "faulty intonation" , "Acuity; Severn port" , "Clarity" .)