Slithery sea creatures (4)
I believe the answer is:
eels
'slithery sea creatures' 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 "(Slippery) fish" , "Water snakes twisting in lees" , "Long thin fish" , "Else they're fish" , "Sea creatures found in lees" .)