Number is happening, watched outside (9)
I believe the answer is:
seventeen
'number' is the definition.
(seventeen is a kind of number)
'happening watched outside' is the wordplay.
'happening' becomes 'event' (I've seen this in other clues).
'watched' becomes 'seen' (I've seen this in another clue).
'outside' indicates putting letters inside.
'event' going within 'seen' is 'SEVENTEEN'.
'is' is the link.
(Other definitions for seventeen that I've seen before include "Age when full driving licence can be obtained" , "Cardinal number" , "Figure" , "Quite a few" , "next clue" .)