The most common and most likely answer to this crossword clue is the 6 letter word KRAKEN
K
R
A
K
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N
More clues leading to the result KRAKEN
- King, dissolute fellow, bit of nasty monster
- Rescue vessel returns with knowledge of monster
- Search minutely in outskirts of Kingston for fabled monster
- Small boy finally saw vessel overturned by legendary sea monster
- Knight carrying implement for legendary sea creature
- Couple of knights about to hunt legendary sea monster
- Nautical measure including trawl for supposed monster
Buddy explains!

The straight part
We know that "monster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monster = kraken.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 KRAKEN mean?
KRAKEN noun- Alternative form of Kraken