Sink filled with fifty fish (8)
I believe the answer is:
flounder
'fish' is the definition.
(flounder is a kind of fish)
'sink filled with fifty' is the wordplay.
'sink' becomes 'founder' (foundering is a kind of sinking).
'filled with' indicates putting letters inside.
'fifty' becomes 'l' (roman numeral).
'founder' going around 'l' is 'FLOUNDER'.
(Other definitions for flounder that I've seen before include "Move clumsily like flatfish?" , "Stagger clumsily in mud (or water)" , "thrash around wildly" , "Be out of one's depth" , "A fish; be out of depth" .)