The most common and most likely answer to this crossword clue is the 6 letter word REBUKE
I've seen this crossword clue in Financial Times
R
E
B
U
K
E
Buddy explains!

The straight part
We know that "Rap on the knuckles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rap on the knuckles = rebuke.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 REBUKE mean?
REBUKE noun- A harsh criticism.
- To criticise harshly; to reprove.