The most common and most likely answer to this crossword clue is the 8 letter word DEALFISH
I've seen this crossword clue in The Times
D
E
A
L
F
I
S
H
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 = dealfish.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 DEALFISH mean?
DEALFISH noun- A deep-sea ribbonfish, Trachipterus arcticus, from the north Atlantic