He had some fabulous ideas (5)
I believe the answer is:
aesop
'he had some fabulous ideas' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for aesop that I've seen before include "very old writer" , "Greek author, famous for his fables" , "Great Greek teller of animal fables" , "A pose of famed fable teller" , "Fabulous writer" .)