Flounder's easily keeping submerged (8)
I believe the answer is:
undersea
'submerged' is the definition.
(I've seen this before)
'flounder's easily keeping' is the wordplay.
'keeping' indicates a hidden word.
'UNDERSEA' can be found hidden inside 'flounders easily'.
(Other definitions for undersea that I've seen before include "in salt water" , "Where bed is" , "submerged" , "Beneath the surface of the ocean" , "Below marine surface" .)