Stone-faced (5)
I believe the answer is:
stoic
'stone-faced' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stoic that I've seen before include "Athenian philosopher" , "Uncomplaining; philosopher" , "One who endures hardship uncomplainingly" , "Resilient philosopher" , "Ancient philosopher" .)