Bill's trendy way of speaking (7)
I believe the answer is:
invoice
'bill's' is the definition.
(invoicing is a kind of billing)
'trendy way of speaking' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'way of speaking' becomes 'voice' (you speak with your voice).
'in'+'voice'='INVOICE'
(Other definitions for invoice that I've seen before include "Statement of sum due" , "Demand for sum due" , "Pollen holder" , "Statement of account" , "List of goods supplied and request for payment" .)