Finally, the say-so comes from the old goddess (3)
I believe the answer is:
eos
'goddess' is the definition.
(I know that Eos is a goddess)
'finally the say-so comes from the old' is the wordplay.
I cannot quite see how this works, but
'from' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
The remaining letters 'es' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eos that I've seen before include "Greek god of the dawn" , "Greek dawn goddess" .)