Warning about constant pressure (5)
I believe the answer is:
force
'pressure' is the definition.
('force' can be a synonym of 'pressure')
'warning about constant' is the wordplay.
'warning' becomes 'fore' (I've seen this in other clues).
'about' indicates putting letters inside.
'constant' becomes 'c' (mathematical abbreviation).
'fore' placed around 'c' is 'FORCE'.
(Other definitions for force that I've seen before include "Strength, power" , "fall" , "Compulsion - police" , "Impetus" , "Powerful effect or influence" .)