Model put up by a fabulous writer (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'model put up by a' is the wordplay.
'model' becomes 'pose' (synonyms).
'put up' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'pose' in reverse letter order is 'esop'.
'esop' after 'a' is 'AESOP'.
(Other definitions for aesop that I've seen before include "Author of many fables" , "A pose by Greek writer of fables" , "satirical writer" , "story teller" , "Poses as ancient Greek writer" .)