Happening to have seen bagging a number (9)
I believe the answer is:
seventeen
'happening' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'seen bagging a number' is the wordplay.
'bagging' means one lot of letters goes inside another (bag can mean to obtain or contain).
'a number' becomes 'event' (I can't explain this - if you can you should give a lot more credence to this answer).
'seen' enclosing '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 "XVII" , "odd number" , "Figure" , "Age when full driving licence can be obtained" , "Quite a few" .)