Recorder reformed Fleet gaol (9)
I believe the answer is:
flageolet
'recorder' is the definition.
(flageolet is a kind of recorder)
'reformed fleet gaol' is the wordplay.
'reformed' indicates anagramming the letters.
'fleet'+'gaol'='fleetgaol'
'fleetgaol' anagrammed gives 'FLAGEOLET'.
(Other definitions for flageolet that I've seen before include "Type of recorder" , "Small flute" , "Pulse" , "bean!" , "musical instrument" .)