Sargasso Sea swimmers (4)
I believe the answer is:
'sargasso sea swimmers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eels that I've seen before include "Lengthy fish" , "Water snakes twisting in lees" , "Long, fleshy fish" , "Snakes found in lees" , "Conger --; jellied --" .)