The most common and most likely answer to this crossword clue is the 11 letter word SURGEONFISH
I've seen this crossword clue in The Times
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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 = surgeonfish.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 SURGEONFISH mean?
SURGEONFISH noun- Any of many species of reef-dwelling fishes, most of them brightly coloured, of the family Acanthuridae. They are named "surgeonfish" because they bear erectile, scalpel-like, dangerously sharp spines on either side of the caudal peduncle.