William's expenses (5)
I believe the answer is:
bills
'william's expenses' is the definition.
I can't judge whether this definition defines the answer.
'william's expenses' is the wordplay.
'william' becomes 'bill'.
'expenses' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'bill'+'s'='BILLS'
Can you help me to learn more?
(Other definitions for bills that I've seen before include "Beaks" , "Invoices" , "Draft laws" .)