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