Flat dwellers in the main struggle (8)
I believe the answer is:
flounder
I believe this clue is a double definition.
'flat dwellers in the main' is the first definition.
(flatfish - 'the main' is a name for the sea)
'struggle' is the second definition.
(floundering is a kind of struggling)
(Other definitions for flounder that I've seen before include "Flatfish - behave awkwardly" , "Have difficulties with the flatfish" , "Grope around" , "thrash around wildly" , "flail" .)