Top bill? Yours truly (4)
I believe the answer is:
acme
'top' is the definition.
('acme' can be a synonym of 'top')
'bill? yours truly' is the wordplay.
'bill?' becomes 'ac'.
'yours truly' becomes 'me'.
'ac'+'me'='ACME'
(Other definitions for acme that I've seen before include "Came awkwardly to the highest point" , "Top; peak" , "highest position" , "Summit, crowning point" , "Summit of achievement" .)