Pose with a retro fabulist (5)
I believe the answer is:
aesop
'fabulist' is the definition.
(Greek composer of fables)
'pose with a retro' is the wordplay.
'with' says to put letters next to each other.
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'pose'+'a'='posea'
'posea' in reverse letter order is 'AESOP'.
(Other definitions for aesop that I've seen before include "Greek author of fables (c 620-560 BC)" , "his writings were fabulous" , "fabulous figure?" , "Poses as ancient Greek writer" , "A pose by Greek writer of fables" .)