Pressure measure (3)
I believe the answer is:
'pressure measure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for psi that I've seen before include "Pounds per square inch (abbr.); Greek letter" , "one employed by Socrates?" , "letter from abroad" , "Pounds per sq. in. (abbr.)" , "Greek character" .)