The most common and most likely answer to this crossword clue is the 4 letter word GOBY
I've seen this crossword clue in Financial Times
G
O
B
Y
More clues leading to the result GOBY
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 = goby.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 GOBY mean?
GOBY noun- Any of various small fish from the large family Gobiidae, in which the pelvic fins are fused to form a disc-shaped sucker.
- A gudgeon, such as Gobio gobio.