Say OK — conclusion drawn from one such as Plato (5)
I believe the answer is:
agree
'say ok' is the definition.
(to say OK to something is to agree to it)
'conclusion drawn from one such as plato' is the wordplay.
'conclusion drawn from' means to remove the last letter (the concluding letter is drawn out).
'one' becomes 'a' (a thing is one thing).
'such as plato' becomes 'Greek' (Plato was Greek).
'a'+'greek'='agreek'
'agreek' with its final letter removed is 'AGREE'.
(Other definitions for agree that I've seen before include "Settle" , "Allow" , "grant" , "Give consent" , "Accord, match" .)