Scope promises to be optimistic (9)
I believe the answer is:
ambitious
'optimistic' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'scope promises' is the wordplay.
'scope' becomes 'ambit' (synonyms).
'promises' becomes 'IOUs' (promises to pay a debt).
'ambit'+'ious'='AMBITIOUS'
'to be' is the link.
Can you help me to learn more?
(Other definitions for ambitious that I've seen before include "Intent on achievements" , "Keen to succeed" , "Aspiring" , "Seeking success" , "Bold - pushy" .)