Grown-up elvers (4)
I believe the answer is:
eels
'grown-up elvers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eels that I've seen before include "(Slippery) fish" , "Food for Cockney" , "Slithery fish" , "Feels small imprisoning water creatures" , "Slippery creatures" .)