It counts against one (5)
I believe the answer is:
debit
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'it counts against' is the wordplay.
'counts' becomes 'deb' (I can't explain this - if you can you should believe this answer much more).
'against' says to put letters next to each other.
'it' after 'deb' is 'DEBIT'.
Can you help me to learn more?
(Other definitions for debit that I've seen before include "Opposite of 23 across" , "Accounting entry showing amount spent" , "Money owed" , "Sum removed from a bank account" , "left-side entry" .)