Swimmer in the Elbe (3)
I believe the answer is:
eel
'swimmer' is the definition.
(eel is a kind of aquatic vertebrate)
'in the elbe' is the wordplay.
'in' indicates a hidden word.
'EEL' can be found hidden inside 'the elbe'.
(Other definitions for eel that I've seen before include "Teleost fish" , "Slender fish" , "Ribbon-like fish" , "Slithery fish" , "Elongated fish" .)