Sit up following ace storyteller (5)
I believe the answer is:
aesop
'storyteller' is the definition.
(Aesop is an example)
'sit up following ace' is the wordplay.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'following' says to put letters next to each other.
'ace' becomes 'a'.
'pose' reversed gives 'esop'.
'esop' after 'a' is 'AESOP'.
(Other definitions for aesop that I've seen before include "his writings were fabulous" , "A pose by Greek writer of fables" , "Ancient Greek fabulist" , "Author of many fables" , "A classical fable teller" .)