The most common and most likely answer to this crossword clue is the 7 letter word NEREIDS
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More clues leading to the result NEREIDS
Buddy explains!

The straight part
We know that "Marine worms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marine worms = nereids.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 NEREIDS mean?
NEREIDS noun- One of 50 sea nymphs who were attendants upon Poseidon (Neptune), and were represented as riding on sea horses, sometimes in human form and sometimes with the tail of a fish.
- A worm of the genus Nereis, having sharp retractable jaws and an annelid body.