Promise to scrub top shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
('ledge' can be a synonym of 'shelf')
'promise to scrub top' is the wordplay.
'promise' becomes 'pledge' (pledge is a kind of promise).
'to scrub top' means to remove the first letter (scrub can mean to remove or erase).
'pledge' with its initial letter taken off is 'LEDGE'.
(Other definitions for ledge that I've seen before include "Shelf-like projection" , "Sill" , "Ridge or shelf" , "A step" , "Projecting shelf, horizontal projection" .)