The most common and most likely answer to this crossword clue is the 8 letter word SARGASSO
I've seen this crossword clue in The Times
S
A
R
G
A
S
S
O
More clues leading to the result SARGASSO
- True stories about river fronts producing algae
- A grass mutates like this into a weed
- Halfwit carried by mythical ship beneath southern sea
- . Some flipping moss, a grass, or seaweed
- Agar’s prepared with source of seaweed like this?
- Donkey on board small ship weed!
- Long story’s true about restricted sea where eels breed
- Gross as a drifting seaweed
- Slimy vegetation across a grassland partly retreating
- Seaweed turned into moss — a grass
Buddy explains!

The straight part
We know that "Seaweed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seaweed = sargasso.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 SARGASSO mean?
SARGASSO noun- A brown alga, of the genus Sargassum, that forms large, floating masses.
- Also Sargasso: a confused, tangled mass or situation.
- A part of an ocean or sea characterized by floating masses of sargassos, like the Sargasso Sea.