Writer one to sit back (5)
I believe the answer is:
aesop
'writer' is the definition.
(I've seen this before)
'one to sit back' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'to sit' becomes 'pose' (as in sitting for a painter).
'back' says the letters should be written in reverse.
'pose' reversed gives 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "Greek teller of animal tales with a moral" , "Ancient Greek fabulist" , "satirical writer" , "Classic fable writer" , "Greek storyteller" .)