It counts against one, being in debt (5)
I believe the answer is:
debit
'debt' is the definition.
(thesaurus)
'it counts against one being in' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'counts' becomes 'deb' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'against' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'being in' means one lot of letters goes inside another.
't' after 'deb' is 'debt'.
'debt' placed around 'i' is 'DEBIT'.
Can you help me to learn more?
(Other definitions for debit that I've seen before include "Sum removed from a bank account" , "Money owed" , "Opposite of credit" , "Record of a sum owed" , "Remove (money) from account" .)