The most common and most likely answer to this crossword clue is the 8 letter word SARDINES
I've seen this crossword clue in The Guardian
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More clues leading to the result SARDINES
Buddy explains!

The straight part
We know that "2/swimmers/2" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 2/swimmers/2 = sardines.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 SARDINES mean?
SARDINES noun- Any one of several species of small herring which are commonly preserved in olive oil or in tins for food, especially the pilchard, or European sardine Sardina pilchardus (syn. Clupea pilchardus). The California sardine Sardinops sagax (syn. Clupea sagax) is similar. The American sardines of the Atlantic coast are mostly the young of the Atlantic herring and of the menhaden.
- Carnelian
- Someone packed or crammed into a small space.