Voice note exposed the French revolutionary (4)
I believe the answer is:
alto
'voice' is the definition.
(alto is a kind of voice)
'note exposed the french revolutionary' is the wordplay.
'exposed' indicates the central letters (the inside of the word is exposed).
'the french' becomes 'la' ('the' in French).
'revolutionary' says the letters should be written backwards (letters revolve around).
The middle of 'note' is 'ot'.
'ot'+'la'='otla'
'otla' written backwards gives 'ALTO'.
(Other definitions for alto that I've seen before include "Low female singing voice" , "The highest adult male singing voice" , "Key old > male who scores high" , "Male singer" , "One who sings" .)