A number having witnessed what happened in (9)
I believe the answer is:
seventeen
'a number' is the definition.
(seventeen is a kind of number)
'witnessed what happened in' is the wordplay.
'witnessed' becomes 'seen' (similar in meaning).
'what happened' becomes 'event' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'seen' placed around 'event' is 'SEVENTEEN'.
'having' is the link.
(Other definitions for seventeen that I've seen before include "Age when full driving licence can be obtained" , "XVII" , "Cardinal number" , "odd number" , "Figure" .)