Vow to put pressure on sill (6)
I believe the answer is:
pledge
'vow' is the definition.
(I know that vow is a kind of pledge)
'pressure on sill' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' means one lot of letters go next to another (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 "Promise, oath" , "Solemn undertaking" , "make toast" , "Give one's word" , "Promise, word of honour" .)