The most common and most likely answer to this crossword clue is the 6 letter word MOCKER
I've seen this crossword clue in The Guardian
M
O
C
K
E
R
More clues leading to the result MOCKER
Buddy explains!

The straight part
We know that "One ridiculing another" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one ridiculing another = mocker.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 MOCKER mean?
MOCKER noun- A person who mocks.
- A mockingbird.
- A deceiver; an impostor.