Half pressure, for certain (4)
I believe the answer is:
sure
'certain' is the definition.
('sure' can be a synonym of 'certain')
'half pressure' is the wordplay.
'half' means to take half the letters.
'pressure' cut in half is 'SURE'.
'for' acts as a link.
(Other definitions for sure that I've seen before include "Definite" , "Certain to be a ruse" , "Convinced" , "See 12" , "Undoubtedly true" .)