Promise to put pressure on sill (6)
I believe the answer is:
pledge
'promise' is the definition.
(pledge is a kind of promise)
'pressure on sill' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'sill' becomes 'ledge' (I've seen this in another clue).
'p'+'ledge'='PLEDGE'
'to put' acts as a link.
(Other definitions for pledge that I've seen before include "Solemn undertaking" , "Vow, affirm" , "Give one's word" , "Deposit" , "assurance" .)