A first-class letter in Greek (5)
I believe the answer is:
alpha
I believe this is a quintuple definition!!
'a' is the first definition.
(the Greek letter A)
'first' is the second definition.
(thesaurus)
'class' is the third definition.
(I have seen 'First class' mean 'alpha' so perhaps 'class' could also mean 'alpha')
'letter' is the fourth definition.
(NATO phonetic alphabet)
'in' acts as a link.
'greek' is the fifth 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 "Greek vowel" , "As heard on radio, a" , "Beginning" , "Brightest star" , "Top grade" .)