Invoice William (4)
I believe the answer is:
bill
This clue is a double definition.
'invoice' is the first definition.
(synonyms)
'william' is the second definition.
(Other definitions for bill that I've seen before include "Invoice; beak" , "List of charges" , "Narrow promontory" , "following 28 [SILVER]" , "- - Clinton or Gates" .)