Sit next to a flipping fabulous writer (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'sit next to a flipping' is the wordplay.
'sit' becomes 'pose' (as in sitting for a painter).
'next to' is a charade indicator (letters next to each other).
'flipping' says the letters should be written in reverse.
'pose'+'a'='posea'
'posea' reversed gives 'AESOP'.
(Other definitions for aesop that I've seen before include "Fabulous writer" , "classic author" , "Great Greek teller of animal fables" , "Greek author of fables (c 620-560 BC)" , "Greek author, famous for his fables" .)