Fabulous writer dishevelled as Poe (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'dishevelled as poe' is the wordplay.
'dishevelled' indicates anagramming the letters.
'as'+'poe'='aspoe'
'aspoe' is an anagram of 'AESOP'.
(Other definitions for aesop that I've seen before include "Author of many fables" , "his writings were fabulous" , "Ancient Greek fabulist" , "Poses as ancient Greek writer" , "Greek father of fables" .)