Plato's "H” (3)
I believe the answer is:
eta
'plato's h' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eta that I've seen before include "European separatists, initially" , "Letter of Greek alphabet" , "Greek letter - Basque nationalist movement" , "Greek letter E; estimated time of arrival (abbr.)" , "Something like 10 [RHO]" .)