Eel (5)
I believe the answer is:
elver
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">eel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(elver is a kind of eel)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for elver that I've seen before include "One may see one upriver" , "Young slippery fish" , "Young swimmer" , "Little fish" , "Immature fish" .)