Christmas no-no? Leak by Wenceslas? (7)

I believe the answer is:

peeking

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'christmas no-no?' is the definition.
(I've seen this in another clue)

'leak by wenceslas?' is the wordplay.
'leak' becomes 'pee' (I've seen this in another clue).
'by' says to put letters next to each other.
'wenceslas?' becomes 'king' (I can't explain this - if you can you should give a lot more credence to this answer).
'pee'+'king'='PEEKING'

Can you help me to learn more?

(Other definitions for peeking that I've seen before include "Looking slyly" , "ooking where one shouldn't" .)

I've seen this clue in The Times and The Sunday Times.

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