P, to Plato (3)
I believe the answer is:
rho
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">p to plato</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 rho that I've seen before include "Gr. letter written as P" , "Greek character" , "Seventeenth letter of the Greek alphabet" , "17th Greek letter" , "Greek P" .)