Order press to cover key letter from Plato (7)
I believe the answer is:
Here is my best explanation:
'letter from plato' is the definition.
(Greek letter so used in Plato's works)
'order press to cover key' is the subsidiary indication.
'order' becomes 'om' (order of merit).
'press' becomes 'iron' (ironing is a kind of pressing**).
'to cover' means one lot of letters goes inside another.
'key' becomes 'c' (I've seen this in other clues).
'iron' going around 'c' is 'icron'.
(Other definitions for omicron that I've seen before include "Greek letter O", "character in Sophocles", "letter from Athens", "Fifteenth letter of the Greek alphabet", "Moronic (anag) - Greek letter", "A rounded character", "Greek short 'o'", "old letter", "Olympic character" and "Greek vowel".)