Exceptional aim? (3,2,1,7)

I believe the answer is:

one in a million

I'm a little stuck... Click here to teach me more about this clue!

'exceptional aim?' is the definition.
The definition suggests a singular noun which matches the answer.

This is all the clue.

Can you help me to learn more?

(Other definitions for one in a million that I've seen before include "Rare or exceptional person or thing" , "Quite exceptional person" , "Gem; exceptional person" , "Wonderful person, almost unique?" , "someone very special? - with 24a" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.