Scope promises to be demanding (9)
I believe the answer is:
ambitious
'be demanding' is the definition.
(synonyms)
'scope promises' is the wordplay.
'scope' becomes 'ambit' (synonyms).
'promises' becomes 'IOUs' (promises to pay a debt).
'ambit'+'ious'='AMBITIOUS'
'to' is the link.
(Other definitions for ambitious that I've seen before include "Aspiring" , "Seeking success" , "Desiring success" , "Having a strong desire for success" , "Keen to succeed" .)