The most common and most likely answer to this crossword clue is the 6 letter word MEDUSA
I've seen this crossword clue in The Times
M
E
D
U
S
A
More clues leading to the result MEDUSA
- This petrified Dutch trapped in tableland
- Monster, terribly amused
- Jellyfish found in Mediterranean and America
- Jellyfish in sea north of America
- Coach amused stinging swimmer, one of 21
- Nation going after sea monster
- Monstrous woman who could be petrifying
- Small Sea States for jellyfish
- Gorgon at sea across the pond
- Monster is wildly amused
Buddy explains!

The straight part
We know that "Jellyfish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jellyfish = medusa.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 MEDUSA mean?
MEDUSA noun- A jellyfish; specifically , a non-polyp form of individual cnidarians, consisting of a gelatinous umbrella-shaped bell and trailing tentacles.