As one was in debt though disputed (8)
I believe the answer is:
reasoned
'as one' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'in debt though disputed' is the wordplay.
'in' indicates putting letters inside.
'debt' becomes 'red' ('in the red' means 'in debt').
'though disputed' becomes 'asone' (I am not sure about this - if you are sure you should believe this answer much more).
'red' enclosing 'asone' is 'REASONED'.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for reasoned that I've seen before include "Deduced or concluded" , "Worked out through logic and sense" , "Performed brain operations" , "sound" , "Debated in calm and logical way" .)