The most common and most likely answer to this crossword clue is the 12 letter word SIPHONOPHORE
S
I
P
H
O
N
O
P
H
O
R
E
Buddy explains!
The straight part
We know that "Marine organism" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marine organism = siphonophore.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 SIPHONOPHORE mean?
SIPHONOPHORE noun- Any of various transparent marine hydrozoans, of the order Siphonophorae, that float or swim as colonies of polyps.