Omega follows it (3)
I believe the answer is:
psi
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">omega follows it</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 "foreign letter" , "one employed by Socrates?" , "Greek letter; pressure unit" , "Twenty-third letter of Greek alphabet" , "Pounds per sq. in. (abbr.)" .)