Letter from Greece
I believe the answer is:
psi
'letter from greece' is the definition.
(letter from the Greek alphabet)
This is all the clue.
(Other definitions for psi that I've seen before include "foreign letter" , "Greek letter; pounds per square inch (abbrev.)" , "Pounds per sq. in. (abbr.)" , "Greek character" , "Greek letter; pressure unit" .)