The most common and most likely answer to this crossword clue is the 8 plus 3 letter word SARGASSO SEA
I've seen this crossword clue in The Guardian
S
A
R
G
A
S
S
O
S
E
A
Buddy explains!
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.