Promise not to use top shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
('ledge' can be a synonym of 'shelf')
'promise not to use top' is the wordplay.
'promise' becomes 'pledge' (pledge is a kind of promise).
'not to use top' means to remove the first letter (the top letter is removed).
'pledge' with its first letter taken off is 'LEDGE'.
(Other definitions for ledge that I've seen before include "Projecting shelf, horizontal projection" , "Submerged underwater shelf" , "Narrow projecting shelf, on cliff or building" , "Shelf-life projection" , "Ridge or shelf" .)