Current endlessly beneath fish (8)
I believe the answer is:
flounder
'fish' is the definition.
(I know that flounder is a type of fish)
'current endlessly beneath' is the wordplay.
'current' becomes 'flow' (synonyms).
'endlessly' means to remove the last letter.
'beneath' becomes 'under' (under means beneath or underneath).
'flow' with its final letter taken away is 'flo'.
'flo'+'under'='FLOUNDER'
(Other definitions for flounder that I've seen before include "swimmer in the sea" , "Fish; stagger clumsily" , "flail" , "Small flatfish" , "Have difficulty" .)