Current endlessly beneath swimmer (8)
I believe the answer is:
flounder
'swimmer' is the definition.
(flounder is a kind of aquatic vertebrate)
'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 removed is 'flo'.
'flo'+'under'='FLOUNDER'
(Other definitions for flounder that I've seen before include "Struggle clumsily - flatfish" , "A fish; be out of depth" , "fumble?" , "Flatfish - behave awkwardly" , "Catch" .)