Half pressure, for certain (4)
I believe the answer is:
sure
'certain' is the definition.
(I know that certain can be written as sure)
'half pressure' is the wordplay.
'half' indicates taking half.
'pressure' halved is 'SURE'.
'for' is the link.
(Other definitions for sure that I've seen before include "Definite" , "Certain to be a ruse" , "Convinced" , "See 12" , "Undoubtedly true" .)