Fish to be all at sea! (8)
I believe the answer is:
flounder
'fish' is the definition.
(I know that flounder is a type of fish)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for flounder that I've seen before include "flail" , "Have difficulties with the flatfish" , "Struggle in water" , "Move clumsily like flatfish?" , "Small flatfish" .)