Police force, one long unpaid? (3,4)
I believe the answer is:
old bill
'police force' is the definition.
(slang term)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for old bill that I've seen before include "forgotten account" , "previous charges" , "Coppers" , "Cops" , "evidence of previous appearances in theatre" .)