Slippery creatures (4)
I believe the answer is:
eels
'slippery creatures' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for eels that I've seen before include "Snakelike fishes" , "They have long bodies and very small fins" , "Sea snakes among lees" , "Long, fleshy fish" , "Water snakes twisting in lees" .)