Letter before omega (3)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">letter before omega</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)