Sneaky, doing time, endless debts to pay (9)
I believe the answer is:
insidious
'sneaky' is the definition.
(I've seen this in another clue)
'doing time endless debts to pay' is the wordplay.
'doing time' becomes 'inside' (I've seen this in another clue).
'endless' says to take the centre.
'debts' becomes 'IOUs' (promises to pay debts).
'to' means one lot of letters go next to another (I've seen this in other clues).
'pay' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
The centre of 'inside' is 'nsid'.
'nsid'+'ious'='nsidious'
'nsidious' put after 'i' is 'INSIDIOUS'.
Can you help me to learn more?
(Other definitions for insidious that I've seen before include "Subtly harmful" , "Beguiling but harmful" , "Slowly having a harmful effect" , "Treacherous; subtle" , "crafty" .)