A crusty letter from Greece, note (3)
I believe the answer is:
pie
'a crusty' is the definition.
I can't judge whether this definition defines the answer.
'letter from greece note' is the wordplay.
'letter from greece' becomes 'pi' (letter from the Greek alphabet).
'note' becomes 'e' (musical note).
'pi'+'e'='PIE'
Can you help me to learn more?
(Other definitions for pie that I've seen before include "Pastry-covered dish" , "Dish baked in an oven, with pastry or mashed potato topping" , "In the sky, it's an unrealistic prospect" , "Fruit or meat dish" , "In the sky, this dish is impractical" .)