Storyteller's written up main work (5)
I believe the answer is:
aesop
'storyteller's' is the definition.
(Aesop is an example)
'written up main work' is the wordplay.
'written up' shows that the letters should be reversed in order.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'work' becomes 'op' (abbreviation for opus).
'sea' backwards is 'aes'.
'aes'+'op'='AESOP'
(Other definitions for aesop that I've seen before include "Greek author, famous for his fables" , "Greek teller of animal tales with a moral" , "his writings were fabulous" , "Greek fable writer" , "Ancient Greek author of moral tales" .)