A Greek letter? (5)
I believe the answer is:
alpha
'a greek letter?' is the definition.
(letter from the Greek alphabet)
This is all the clue.
(Other definitions for alpha that I've seen before include "Foremost" , "Brilliant star" , "First in order of importance (for a Greek?)" , "A for Aristophanes!" , "Early testing stage for new software or hardware" .)