Thrash about (5)
'thrash about' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for flail that I've seen before include "Thresh about" , "thrash around" , "Wave about wildly" , "Weapon; swing wildly" , "Threshing device" .)
'thrash about' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for flounder that I've seen before include "fumble?" , "flail" , "swimmer in the sea" , "Thrash about helplessly around the fish" , "Stagger clumsily in mud (or water)" .)