The most common and most likely answer to this crossword clue is the 6 letter word IMPUGN
I've seen this crossword clue in The Guardian
I
M
P
U
G
N
More clues leading to the result IMPUGN
- Challenge on reflection no good: how to go from London to Leeds?
- Dispute the accuracy of little devil with damaged gun
- Criticise unopened dumping ground
- Setter’s dog facing new attack
- Troublemaker organising gnu attack
- Hang up mirror section, backing challenge
- Challenge “this setter’s a dog” notion, initially
- Criticise “One Man and His Dog” on a point
- One man chucking article for dog to have a go at
- Rogue characters regularly seen in surging attack
Buddy explains!

The straight part
We know that "Criticise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case criticise = impugn.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 IMPUGN mean?
IMPUGN verb- To assault, attack.
- To verbally assault, especially to argue against an opinion, motive, or action; to question the truth or validity of.