For a writer of old, a set-back (5)
I believe the answer is:
aesop
'for a writer' is the definition.
(I've seen this before)
'a set-back' is the wordplay.
'set' becomes 'pose' (synonyms).
'back' says the letters should be written in reverse.
'pose' written backwards gives 'esop'.
'a'+'esop'='AESOP'
'of old' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for aesop that I've seen before include "very old writer" , "fabulous figure?" , "So ape the writer of fables" , "he wrote in Greek" , "satirical writer" .)