Promise not to start with windowsill (5)
I believe the answer is:
ledge
'windowsill' is the definition.
(I've seen this in another clue)
'promise not to start' is the wordplay.
'promise' becomes 'pledge' (pledge is a kind of promise).
'not to start' means to remove the first letter.
'pledge' with its first letter taken off is 'LEDGE'.
'with' is the link.
(Other definitions for ledge that I've seen before include "Mantelpiece" , "Narrow projection or shelf" , "Projecting shelf, horizontal projection" , "found halfway up a cliff?" , "Surface under e.g. window" .)