Ancient storyteller made avant-garde leader sit up (5)
I believe the answer is:
aesop
'ancient storyteller' is the definition.
(I've seen this in another clue)
'avant-garde leader sit up' is the wordplay.
'avant-garde leader' becomes 'a' (I can't explain this - if you can you should believe this answer much more).
'sit' becomes 'pose' (as in sitting for a painter).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'pose' written backwards gives 'esop'.
'a'+'esop'='AESOP'
'made' acts as a link.
Can you help me to learn more?
(Other definitions for aesop that I've seen before include "Ancient Greek fabulist" , "Great Greek teller of animal fables" , "Greek author, famous for his fables" , "Fabulous man" , "Author of many fables" .)