Sleeping policeman in Knock? (4)

I believe the answer is:

bump

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

'knock?' is the definition.
(I know that bump can be written as bump)

'sleeping policeman' is the wordplay.
'sleeping' becomes 'bu' (I can't justify this - if you can you should believe this answer much more).
'policeman' becomes 'MP' (abbreviation for Military Policeman).
'bu'+'mp'='BUMP'

'in' is the link.

Can you help me to learn more?

(Other definitions for bump that I've seen before include "Hit" , "Jolt - collision - swelling" , "Jostle" , "Collide - protuberance" , "Jolting movement or skin swelling" .)

I've seen this clue in The Irish Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.