Fabulist initially allowed revolutionary attitude (5)
I believe the answer is:
aesop
'fabulist' is the definition.
(Greek composer of fables)
'initially allowed revolutionary attitude' is the wordplay.
'initially' says to take the initial letters.
'revolutionary' shows that the letters should be reversed in order (letters revolve around).
'attitude' becomes 'pose' (attitude can mean a pose or stance).
The first letter of 'allowed' is 'a'.
'pose' backwards is 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "Ancient Greek author of moral tales" , "Greek author of fables (c 620-560 BC)" , "A classical fable teller" , "So ape the writer of fables" , "Greek storyteller" .)