A letter to a Greek (5)
I believe the answer is:
alpha
I believe this is a quadruple definition!!
'a' is the first definition.
(the Greek letter A)
'letter' is the second definition.
(NATO phonetic alphabet)
'to a' is the third definition.
(the Greek letter A)
'greek' is the fourth definition.
(I have seen 'A Greek' mean 'alpha' so perhaps 'greek' could also mean 'alpha')
(Other definitions for alpha that I've seen before include "Top grade" , "Top star" , "First-rate" , "Burrowing American rodent" , "star outshining the others" .)