From him, sour grapes were just a bit of a story (5)
I believe the answer is:
aesop
'from him sour grapes were' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a bit of a story' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
'bit' could be 'sop' (sop is a kind of bit) and 'sop' is present in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'just' acts as a link.
Can you help me to learn more?
(Other definitions for aesop that I've seen before include "Greek author of fables (c 620-560 BC)" , "A pose by Greek writer of fables" , "Ancient Greek author of moral tales" , "satirical writer" , "A pose of famed fable teller" .)