Is taken aback by opening of grand master's letter from Greece (5)
I believe the answer is:
sigma
'letter from greece' is the definition.
(letter from the Greek alphabet)
'is taken aback by opening of grand master's' is the wordplay.
'taken aback' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'opening of' says to take the initial letters.
'master' becomes 'ma' (Master of Arts).
The first letter of 'grand' is 'g'.
'is' written backwards gives 'si'.
'si'+'g'+'ma'='SIGMA'
(Other definitions for sigma that I've seen before include "character in Athens" , "Allowing no relaxation" , "Greek letter used as summation sign" , "Athenian character" , "Greek letter corresponding to S" .)