Omega preceder (3)
I believe the answer is:
psi
'omega preceder' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for psi that I've seen before include "Twenty-third letter of Greek alphabet" , "letter from abroad" , "one employed by Socrates?" , "Pounds per sq. in. (abbr.)" , "foreign letter" .)