The most common and most likely answer to this crossword clue is the 9 letter word PIKEPERCH
I've seen this crossword clue in The Guardian
P
I
K
E
P
E
R
C
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 = pikeperch.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 PIKEPERCH mean?
PIKEPERCH noun- Any of the five species of fish in the genus Sander of the family Percidae (perches and darters).