Fabulist initially adopted revolutionary attitude (5)
I believe the answer is:
aesop
'fabulist' is the definition.
(Greek composer of fables)
'initially adopted revolutionary attitude' is the wordplay.
'initially' indicates taking the first letters.
'revolutionary' is a reversal indicator.
'attitude' becomes 'pose' (attitude can mean a pose or stance).
The first letter of 'adopted' is 'a'.
'pose' written backwards gives 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "Great Greek teller of animal fables" , "Fable-teller" , "fabulous figure?" , "Greek storyteller" , "Fabulous man" .)