Letter between phi and psi (3)
I believe the answer is:
chi
'letter between phi and psi' 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 "Chinese life-force" , "The life force, according to traditional Chinese medicine" , "Greek letter; Chinese life force" , "One character abroad" , "foreign character" .)