The most common and most likely answer to this crossword clue is the 9 letter word ANGELFISH
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More clues leading to the result ANGELFISH
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 = angelfish.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 ANGELFISH mean?
ANGELFISH noun- A freshwater fish, tropical cichlids of the genus Pterophyllum.
- A marine fish of the family Pomacanthidae, common on shallow tropical reefs.