Third-to-last Greek letter (3)
I believe the answer is:
chi
'third-to-last greek letter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for chi that I've seen before include "In Chinese medicine, the life force inherent in all things" , "Twenty-second Greek letter" , "Letter written by European" , "foreign character" , "Classical symbol" .)