A model pops up for fabulist (5)
I believe the answer is:
aesop
'fabulist' is the definition.
(Greek composer of fables)
'a model pops up' is the wordplay.
'model' becomes 'pose' ('pose' can be a synonym of 'model'**).
'pops up' says the letters should be written in reverse (in a down clue, letters go up).
'pose' back-to-front is 'esop'.
'a'+'esop'='AESOP'
'for' acts as a link.
(Other definitions for aesop that I've seen before include "his writings were fabulous" , "classic author" , "very old writer" , "Great Greek teller of animal fables" , "A pose of famed fable teller" .)