Serpentine swimmer (3)
I believe the answer is:
eel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">serpentine 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 "Slippery creature" , "Moray or electric, for instance" , "Teleost fish" , "slipper" , "Apode" .)