Musician's device satisfied small number in capital (9)
I believe the answer is:
metronome
'musician's device' is the definition.
(I know that a metronome is a typically used by a musician)
'satisfied small number in capital' is the wordplay.
'satisfied' becomes 'met' (as in 'met the conditions of his contract').
'small number' becomes 'no' (abbreviation for 'number'; 'small' suggests an abbreviation).
'in' indicates putting letters inside.
'capital' becomes 'Rome' (capital of Italy).
'no' inserted within 'rome' is 'ronome'.
'met'+'ronome'='METRONOME'
(Other definitions for metronome that I've seen before include "Device to indicate tempo" , "Device for keeping time" , "Pendulum to indicate tempo of music" , "Musician's time device" , "Musical timekeeper" .)