The most common and most likely answer to this crossword clue is the 5 letter word CISCO
I've seen this crossword clue in The Times
C
I
S
C
O
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 = cisco.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 CISCO mean?
CISCO noun- Any North American freshwater fish of certain species of the genus Coregonus that live in cold-water lakes.