First-class letter (5)
I believe the answer is:
alpha
This clue is a triple definition!
'first' is the first definition.
(thesaurus)
'class' is the second definition.
(I have seen 'First class' mean 'alpha' so perhaps 'class' could also mean 'alpha')
'letter' is the third definition.
(NATO phonetic alphabet)
(Other definitions for alpha that I've seen before include "Leading Greek character" , "Brilliant star" , "Early testing stage for new software or hardware" , "First letter of the Greek alphabet" , "- - and Omega" .)