Promise to be quiet on sill (6)
I believe the answer is:
pledge
'promise' is the definition.
(I know this)
'quiet on sill' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'on' means one lot of letters go next to another.
'sill' becomes 'ledge' (I've seen this in another clue).
'p'+'ledge'='PLEDGE'
'to be' is the link.
(Other definitions for pledge that I've seen before include "Plight" , "make toast" , "Firm commitment" , "Covenant" , "Bond" .)