The most common and most likely answer to this crossword clue is the 8 letter word STARFISH
I've seen this crossword clue in The Daily Telegraph
S
T
A
R
F
I
S
H
More clues leading to the result STARFISH
- Supposing Romeo’s turned in hidden treasure to sea dweller
- Spooner’s somewhat quick marine creature
- Fir moved into store that has several arms, mainly
- His raft’s ruffled sea creature
- Celebrity female is accompanying husband, a swimmer
- Marine creature sounds Polish or sticky to some.
- A swimmer has first to get changed
- Sea creature has first to swim
- As seen in the constellation Pisces?
- Heavily armed sea creature wrecked his rafts
Buddy explains!

The straight part
We know that "marine creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marine creature = starfish.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 STARFISH mean?
STARFISH noun- Any of various asteroids or other echinoderms (not in fact fish) with usually five arms, many of which eat bivalves or corals by everting their stomach.
- Any many-armed or tentacled sea invertebrate, whether cnidarian, echinoderm, or cephalopod.
- A woman who reluctantly takes part in sexual intercourse, and lays on her back while spreading her limbs.
- (usually in translations of Japanese pornography) The anus.
- To assume a splayed-out shape, like that of a starfish.
- To form into a splayed-out shape, like that of a starfish.