Swimmer feeling around (3)
I believe the answer is:
eel
'swimmer' is the definition.
(eel is a kind of aquatic vertebrate)
'feeling around' is the wordplay.
'around' indicates a hidden word.
'EEL' can be found hidden inside 'feeling'.
(Other definitions for eel that I've seen before include "Long thin fish" , "A sea snake" , "slipper" , "Electric swimmer, perhaps?" , "Slippery swimmer (may be jellied)" .)