The most common and most likely answer to this crossword clue is the 5 letter word BIFFS
B
I
F
F
S
More clues leading to the result BIFFS
Buddy explains!

The straight part
We know that "Knocks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knocks = biffs.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BIFFS mean?
BIFFS noun- A sudden, sharp blow or punch.
- A wipeout.
- To punch or hit.
- To discard; to throw out; to throw away.
- To wipe out; to faceplant; to fall.