Fabulist initially adopted backward attitude (5)
I believe the answer is:
aesop
'fabulist' is the definition.
(Greek composer of fables)
'initially adopted backward attitude' is the wordplay.
'initially' says to take the initial letters.
'backward' says the letters should be written in reverse.
'attitude' becomes 'pose' (I've seen this before).
The first letter of 'adopted' is 'a'.
'pose' back-to-front is 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "classic author" , "satirical writer" , "A pose of famed fable teller" , "Fabulous man" , "Greek teller of animal tales with a moral" .)