Sink filled with large fish (8)
I believe the answer is:
flounder
'fish' is the definition.
(I know that flounder is a type of fish)
'sink filled with large' is the wordplay.
'sink' becomes 'founder' (founder can mean to fill with water and sink).
'filled with' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'founder' enclosing 'l' is 'FLOUNDER'.
(Other definitions for flounder that I've seen before include "Struggle clumsily - flatfish" , "Flatfish - behave awkwardly" , "Predatory fish" , "A fish; be out of depth" , "Catch" .)