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 can't justify this - if you can you should give a lot more credence to this answer).
'against' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'being in' is an insertion indicator.
't' after 'deb' is 'debt'.
'debt' going 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" .)