Young eel (5)
I believe the answer is:
elver
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">young eel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(an elver is a young eel)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for elver that I've seen before include "A baby eel" , "Immature eel" , "A young 11" , "Slippery youngster" , "Young slippery fish" .)