Papers penned by revolutionary left-wing idealist (7)
I believe the answer is:
dreamer
'idealist' is the definition.
(I know that idealist can be written as dreamer)
'papers penned by revolutionary left-wing' is the wordplay.
'papers' becomes 'ream' (a ream is a large amount of paper).
'penned by' indicates putting letters inside.
'revolutionary' says the letters should be written backwards.
'left wing' becomes 'red' (red can informally mean communist).
'red' in reverse letter order is 'der'.
'ream' going into 'der' is 'DREAMER'.
(Other definitions for dreamer that I've seen before include "Idealist - visionary" , "Fantasist" , "Gerontius" , "such an unpractical person!" , "stargazer" .)