Beginner in Athens (5)
I believe the answer is:
alpha
'beginner in athens' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
This is the entire clue.
(Other definitions for alpha that I've seen before include "very good rating" , "Greek vowel" , "Foremost" , "First-rate" , "best score" .)