Fish struggle to swim? (8)
I believe the answer is:
flounder
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 "Flatfish - behave awkwardly" , "Stagger clumsily in mud (or water)" , "Catch" , "flail" , "Fish -- stumble helplessly" .)