Fish struggle to swim? (8)
This clue is a double definition.
'fish' is the first definition.
(flounder is a kind of fish)
'struggle to swim?' is the second definition.
(to flounder is to struggle in water)
(Other definitions for flounder that I've seen before include "thrash around wildly", "swimmer in the sea", "Fish -- stumble helplessly", "Flatfish - behave awkwardly", "Small flatfish".)