Unusually heartless, collecting proof of debts in a horrible manner (8)
I believe the answer is:
odiously
'in a horrible manner' is the definition.
I can't judge whether this definition defines the answer.
'unusually heartless collecting proof of debts' is the wordplay.
'unusually' becomes 'oddly' (both can mean strangely).
'heartless' means to remove the middle letters.
'collecting' means one lot of letters goes inside another.
'proof of debts' becomes 'ious' (I can't explain this - if you can you should give a lot more credence to this answer).
'oddly' with its middle removed is 'odly'.
'odly' enclosing 'ious' is 'ODIOUSLY'.
Can you help me to learn more?
(Other definitions for odiously that I've seen before include "In a repulsive manner" , "in a repulsive way" .)