Some revolutionary tracts are popular works (6)
I believe the answer is:
operas
'works' is the definition.
(I have seen 'Musical works' mean 'operas' so perhaps 'works' could also mean 'operas')
'some revolutionary tracts are popular' is the wordplay.
'revolutionary' says the letters should be written in reverse (letters revolve around).
'some' says the answer is hidden in the clue (some parts of the word are used).
'sarepo' is hidden in the letters of 'tracts are popular'.
'sarepo' in reverse letter order is 'OPERAS'.
(Other definitions for operas that I've seen before include "Dramas set to music" , "Musical performances" , "eg Faust, Salome" , "Stage dramas" , "Musical plays" .)