A veto on twelve (4)

I believe the answer is:

noon

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

'twelve' is the definition.
(twelve noon)

'a veto on' is the wordplay.
'a veto' becomes 'no' (I can't explain this - if you can you should believe this answer much more).
'no'+'on'='NOON'

Can you help me to learn more?

(Other definitions for noon that I've seen before include "12 o'clock" , "See 8" , "Twelve during daylight" , "A moment of time" , "The middle of the day" .)

I've seen this clue in The Irish News.

Want a hint initially instead of a full solution? Install my app