Crossword Genius

A for Aristotle (5)

Ross

I believe the answer is:

alpha

'a' is the definition.
(the Greek letter A)

I can't explain the rest of the clue.

(Other definitions for alpha that I've seen before include "high mark" , "Beginning" , "very good rating" , "Greek vowel" , "Foremost" .)

I've seen this clue in The Guardian.
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