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' suggests taking the first letters.
'revolutionary' says the letters should be written in reverse.
'attitude' becomes 'pose' (I've seen this before).
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" .)