Fabulous writer with a revolutionary attitude (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'with a revolutionary attitude' is the wordplay.
'with' is a charade indicator (letters next to each other).
'revolutionary' says the letters should be written backwards.
'attitude' becomes 'pose' (I've seen this before).
'pose' backwards is 'esop'.
'a' next to 'esop' is 'AESOP'.
(Other definitions for aesop that I've seen before include "A classical fable teller" , "A pose of famed fable teller" , "Classic fable writer" , "Poses as ancient Greek writer" , "Fable-teller" .)