Front put up by a fabulous Greek writer (5)
I believe the answer is:
aesop
'fabulous greek writer' is the definition.
(writer of fables)
'front put up by a' is the wordplay.
'front' becomes 'pose' (I've seen this before).
'put up' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'pose' backwards is 'esop'.
'esop' after 'a' is 'AESOP'.
(Other definitions for aesop that I've seen before include "satirical writer" , "Author of many fables" , "story teller" , "his writings were fabulous" , "Greek storyteller" .)