Plato's "P" (3)
I believe the answer is:
rho
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">plato's p</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rho that I've seen before include "Sophocles character" , "Seventeenth letter of the Greek alphabet" , "Greek character" , "Greek letter R" , "Something like 25 [ETA]" .)