Say more than anyone else— to the greatest degree (9)
I believe the answer is:
uttermost
'say more than' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'anyone else-to the greatest degree' is the wordplay.
'anyone else' becomes 'utter' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to the greatest degree' becomes 'most' (I've seen this in another clue).
'utter'+'most'='UTTERMOST'
Can you help me to learn more?
(Other definitions for uttermost that I've seen before include "Chief" , "Last" , "best" , "Greatest possible extent" , "Extreme" .)