Note revolutionary chasing unknown soul (6)
I believe the answer is:
psyche
'soul' is the definition.
('psyche' can be a synonym of 'soul')
'note revolutionary chasing unknown' is the wordplay.
'note' becomes 'ps' (PS is an example**).
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary).
'chasing' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'che' put after 'y' is 'yche'.
'ps'+'yche'='PSYCHE'
(Other definitions for psyche that I've seen before include "Princess beloved by Cupid" , "The mind or Cupid's lover" , "Eros's lover" , "Core of one's personality" , "Inner self" .)