Slithery swimmer (3)
I believe the answer is:
eel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">slithery swimmer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for eel that I've seen before include "Long thin fish" , "Electric swimmer, perhaps?" , "Slippery figure" , "Snakelike fish" , "Slippery character" .)