Letter before omega (3)
I believe the answer is:
'letter before omega' 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 "Twenty-third letter of Greek alphabet" , "Greek letter; pounds per square inch (abbrev.)" , "one employed by Socrates?" , "Pounds per square inch (abbr.); Greek letter" , "foreign letter" .)