Is this your crossword clue?
"knock" is also used as a crossword clue. If that's what you saw, the answer is often rap (3 letters).
Definitions
the act of hitting vigorously
Used in a sentence
“It came to me, like a knock on the door / Erasing all that had gone before / Broken wings can be mended / But this love of ours, we've got to defend it”
“He took a knock on the head.”
“"Come on!" cried Mr. Beaver, who was almost dancing with delight. "Come and see! This is a nasty knock for the Witch! It looks as if her power was already crumbling."”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Tap at a door”5 letters
02“Rap on a door”5 letters
Not quite right?
"Search similar patterns."
K____