Attitude rejected by a fabulous writer (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'attitude rejected by a' is the wordplay.
'attitude' becomes 'pose' (I've seen this before).
'rejected' is a reversal indicator.
'by' says to put letters next to each other.
'pose' in reverse letter order is 'esop'.
'esop' put after 'a' is 'AESOP'.
(Other definitions for aesop that I've seen before include "Author of many fables" , "Fabulous writer" , "Ancient Greek fabulist" , "Greek teller of animal tales with a moral" , "Greek author of fables (c 620-560 BC)" .)