Crossword Genius

Happening to have noticed, on the outside, a number (9)

Ross

I believe the answer is:

seventeen

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

'happening' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'noticed on the outside a number' is the wordplay.
'noticed' becomes 'seen' (I've seen this in other clues).
'on the outside' is an insertion indicator.
'a number' becomes 'event' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'seen' going around 'event' is 'SEVENTEEN'.

'to have' acts as a link.

Can you help me to learn more?

(Other definitions for seventeen that I've seen before include "Figure" , "Cardinal number" , "odd number" , "Quite a few" , "next clue" .)

I've seen this clue in the Evening Standard.
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