Plato's "T” (3)
I believe the answer is:
tau
'plato's t' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tau that I've seen before include "Letter from Greece" , "A subatomic particle" , "letter from foreigner" , "Homeric character" , "19th letter of the Greek alphabet" .)