Some stamps identifying a letter from Greece (3)
I believe the answer is:
psi
'letter from greece' is the definition.
(letter from the Greek alphabet)
'some stamps identifying a' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'PSI' is hidden in the letters of 'stamps identifying a'.
(Other definitions for psi that I've seen before include "one employed by Socrates?" , "foreign letter" , "letter from abroad" , "Greek letter; pressure unit" , "Pounds per square inch (abbr.); Greek letter" .)