Cryptic crossword clue
Swimmer caught the flu — it’s bad 10

The most common and most likely answer to this crossword clue is the 10 letter word CUTTLEFISH

I've seen this crossword clue in The Daily Telegraph
C
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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 = cuttlefish.
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 CUTTLEFISH mean?

CUTTLEFISH noun
  1. Any of various squid-like cephalopods (marine mollusks) of the order Sepiida that have eight arms, two retractable tentacles, and a calcareous internal shell, and can eject a dark ink when threatened

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