A model upset Greek writer (5)
I believe the answer is:
aesop
'greek writer' is the definition.
(I've seen this before)
'a model upset' is the wordplay.
'model' becomes 'pose' (model can mean to pose).
'upset' shows that the letters should be reversed in order.
'pose' reversed gives 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "Ancient Greek author of moral tales" , "he wrote in Greek" , "A pose by Greek writer of fables" , "very old writer" , "Greek author of fables (c 620-560 BC)" .)