Storyteller first-class model upset (5)
I believe the answer is:
aesop
'storyteller' is the definition.
(Aesop is an example)
'first-class model upset' is the wordplay.
'first-class' becomes 'A' (as in A-list, A-team etc).
'model' becomes 'pose' (model can mean to pose).
'upset' says the letters should be written in reverse.
'pose' written backwards gives 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "Ancient Greek author of moral tales" , "Author of many fables" , "satirical writer" , "Greek teller of animal tales with a moral" , "Greek storyteller" .)