Advertising expenditure? (5)
I believe the answer is:
bills
'advertising expenditure?' is the definition.
Although both the answer and 'expenditure?' are nouns, they are not in the same form of the noun.
'advertising expenditure?' is the wordplay.
'advertising' becomes 'bill' (bill is a kind of advertising).
'expenditure?' becomes 's' (I can't justify 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" .)