A slippery youngster (5)
I believe the answer is:
elver
'a slippery youngster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for elver that I've seen before include "Young slippery fish" , "Young swimmer" , "slippery little thing" , "Slippery youngster" , "Young, migrating eel" .)