The most common and most likely answer to this crossword clue is the 6 letter word FERULE
I've seen this crossword clue in The Guardian
F
E
R
U
L
E
More clues leading to the result FERULE
Buddy explains!

The straight part
We know that "punishment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case punishment = ferule.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 FERULE mean?
FERULE noun- A ruler-shaped instrument, generally used to slap naughty children on the hand.
- To punish with a ferule.