X, to Greeks (3)
I believe the answer is:
chi
'x to greeks' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for chi that I've seen before include "T'ai -, martial art" , "Mental projection" , "foreign character" , "In Chinese medicine, the life force inherent in all things" , "Classical character" .)