Some reef dwellers (4)
I believe the answer is:
eels
'some reef dwellers' 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 types" , "Elongated fish" , "Sea snakes among lees" , "Elongated fishes" , "Else (anag.)" .)