Happening to have noticed, on the outside, a number (9)
I believe the answer is:
seventeen
'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" .)