The most common and most likely answer to this crossword clue is the 3 letter word JAB
I've seen this crossword clue in The Times
J
A
B
Buddy explains!

The straight part
We know that "critical comment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case critical comment = jab.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 JAB mean?
JAB noun- A quick stab or blow; a poking or thrusting motion.
- A short straight punch.
- A medical injection.
- A vaccination, whether or not delivered via conventional injection.
- A mild verbal insult.
- To poke or thrust abruptly, or to make such a motion.
- To deliver a quick punch.
- To give someone an injection