Twisting, concealing promises to return unconventionally (9)
I believe the answer is:
curiously
'to return unconventionally' is the definition.
I can't tell whether this defines the answer.
'twisting concealing promises' is the wordplay.
'twisting' becomes 'curly' (I am not sure about this - if you are sure you should believe this answer much more).
'concealing' means one lot of letters goes inside another.
'promises' becomes 'IOUs' (promises to pay a debt).
'curly' going around 'ious' is 'CURIOUSLY'.
Can you help me to learn more?
(Other definitions for curiously that I've seen before include "strangely" , "Inquisitive types behave so" , "How questions are asked" , "Oddly, peculiarly" , "In an odd way" .)