Sit with a backward old storyteller (5)
I believe the answer is:
aesop
'old storyteller' is the definition.
(I know that Aesop is old)
'sit with a backward' is the wordplay.
'sit' becomes 'pose' (as in sitting for a painter).
'with' says to put letters next to each other.
'backward' shows that the letters should be reversed in order.
'pose'+'a'='posea'
'posea' reversed gives 'AESOP'.
(Other definitions for aesop that I've seen before include "he wrote in Greek" , "Great Greek teller of animal fables" , "classic author" , "satirical writer" , "Ancient Greek fabulist" .)