The most common and most likely answer to this crossword clue is the 10 letter word RIBBONFISH
I've seen this crossword clue in The Times
R
I
B
B
O
N
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 = ribbonfish.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 RIBBONFISH mean?
RIBBONFISH noun- Any of several lampriform fish, of the family Trachipteridae, having long, ribbon-like bodies.