The most common and most likely answer to this crossword clue is the 5 letter word SQUID
I've seen this crossword clue in The Daily Telegraph
S
Q
U
I
D
More clues leading to the result SQUID
- Seafood’s twenty-one shillings?
- Small amount of money that’s found in the sea
- £6for seafood
- Old Bob to pound sea creature
- Pound backs dollar
- Molluscs initially shelled by Parisian who died
- Marine life in Guinea?
- One shilling, then twenty times more animals
- Swimmer is one taking over from ace in team
- Skip last couple of bits of pulpy, wet seafood
Buddy explains!

The straight part
We know that "Swimmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swimmer = squid.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 SQUID mean?
SQUID noun- Any of several carnivorous marine cephalopod mollusks, of the order Teuthida, having a mantle, eight arms, and a pair of tentacles
- A fishhook with a piece of bright lead, bone, or other substance fastened on its shank to imitate a squid.
- (mildly) A sailor in the Navy.
- A quid; one pound sterling.
- To fish with the kind of hook called a squid.
- (parachuting) To cause squidding (an improper, partial, parachute inflation, that results in the sides of the parachute folding in on the center, and pulsating back and forth).
- A motorcyclist, especially a sport biker, characterized by reckless riding and lack of protective gear.