2 swimmers (8)
I believe the answer is:
sardines
'swimmers' is the definition.
(sardine is a kind of aquatic vertebrate)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sardines that I've seen before include "Squash-together party game" , "Cumulative hiding game" , "Small fish, usually canned" , "Aridness of small fish" , "Children's game - fish" .)