Order press to cover key letter from Plato (7)
I believe the answer is:
omicron
'letter from plato' is the definition.
(Greek letter so used in Plato's works)
'order press to cover key' is the wordplay.
'order' becomes 'om' (order of merit).
'press' becomes 'iron' (ironing is a kind of pressing).
'to cover' is an insertion indicator.
'key' becomes 'c' (I've seen this in other clues**).
'iron' enclosing 'c' is 'icron'.
'om'+'icron'='OMICRON'
(Other definitions for omicron that I've seen before include "Round character" , "Olympic character" , "character from Athens" , "Moronic (anag) - Greek letter" , "Greek vowel" .)